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My car has developed a strange knock. :fighting0025:

I can only hear it a hot idle with load applied, i.e. AC on and/or drive selected. Even the slightest raise of rpm makes the sound go away.

It sounds like a steel bar tapping at a 1/8 inch steel sheet or something like that.

The sound is coming from the front cylinder bank somewhere between the front engine mount and the compressor. I have used a stethoscope but have not been able to determine the origin of the sound. The knock has the same frequency as the camshaft.

The engine is running strong except from a slight roughness at idle that it has had forever (I had a vacuum leak but it seems like I still have a slight leak somewhere which I will hunt down eventually).

If I turn the AC off and let the car idle in park the car is running as silent as it should. With the AC on or in drive however it sounds like it is going to come apart any second.

What could cause this sound? No codes.

I was thinking lifters but they wouldn't be dependent on engine load. Cold carbon rap would be worse with a cold engine. Spark knock?

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It sounds like little like a main bearing knock from the symptoms but the sound is not a thunk but a rap. I would look at the damper wheel and pulley, and at the serpentine belt.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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It sounds like little like a main bearing knock from the symptoms but the sound is not a thunk but a rap. I would look at the damper wheel and pulley, and at the serpentine belt.

Thanks for the input!

The balancer is wobbling a bit.

I'll try with the belt of and with a hot engine. Strange.

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a/c on does increase engine load slightly. but motor knocks with a/c off and in gear? do not know if it is serp belt dependent? quiet at idle, in park. maybe motor mount allowing engine to shift slightly?

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It sounds like little like a main bearing knock from the symptoms but the sound is not a thunk but a rap. I would look at the damper wheel and pulley, and at the serpentine belt.

Thanks for the input!

The balancer is wobbling a bit.

I'll try with the belt of and with a hot engine. Strange.

Mark the balancer outer and inner parts separated with a rubber ring to see whether they move with respect to each other. Use a nail polish for marking.

Could be a spark knock as well. EGR is disabled in park. Check your EGR valve for carbon deposits, free movement of the plunger, integrity of the diaphragm. The latter is easy to test right on the car. Pull the vacuum line off the valve, depress the diaphragm with finger all the way and plug the vacuum input with finger. If the diaphragm moves back, replace the valve.

The saddest thing in life is wasted talent

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It sounds like little like a main bearing knock from the symptoms but the sound is not a thunk but a rap. I would look at the damper wheel and pulley, and at the serpentine belt.

Thanks for the input!

The balancer is wobbling a bit.

I'll try with the belt of and with a hot engine. Strange.

Mark the balancer outer and inner parts separated with a rubber ring to see whether they move with respect to each other. Use a nail polish for marking.

Could be a spark knock as well. EGR is disabled in park. Check your EGR valve for carbon deposits, free movement of the plunger, integrity of the diaphragm. The latter is easy to test right on the car. Pull the vacuum line off the valve, depress the diaphragm with finger all the way and plug the vacuum input with finger. If the diaphragm moves back, replace the valve.

look1-1-1.jpg
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It sounds like little like a main bearing knock from the symptoms but the sound is not a thunk but a rap. I would look at the damper wheel and pulley, and at the serpentine belt.

Thanks for the input!

The balancer is wobbling a bit.

I'll try with the belt of and with a hot engine. Strange.

Mark the balancer outer and inner parts separated with a rubber ring to see whether they move with respect to each other. Use a nail polish for marking.

Could be a spark knock as well. EGR is disabled in park. Check your EGR valve for carbon deposits, free movement of the plunger, integrity of the diaphragm. The latter is easy to test right on the car. Pull the vacuum line off the valve, depress the diaphragm with finger all the way and plug the vacuum input with finger. If the diaphragm moves back, replace the valve.

Addalak, he has an 02 STS..

look1-1-1.jpg
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Thanks for the input!

The balancer is wobbling a bit.

I'll try with the belt of and with a hot engine. Strange.

I am not sure if I think of the right part, but if the balancer you mention is the same as the "main serpentine belt drive pulley" (the big one that is connected to the crankshaft on the engine) it is normal for it to look wobbly. The drive pulley is thicker on one side. The first time I saw this, I thought something was wrong, but a Cadillac tech told me why it looks like that (it is to balance it), and that it is normal. Obviously, the grooves where the belt goes are not suppose to wobble - only the side of the pulley.

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It sounds like little like a main bearing knock from the symptoms but the sound is not a thunk but a rap. I would look at the damper wheel and pulley, and at the serpentine belt.

Thanks for the input!

The balancer is wobbling a bit.

I'll try with the belt of and with a hot engine. Strange.

Mark the balancer outer and inner parts separated with a rubber ring to see whether they move with respect to each other. Use a nail polish for marking.

Could be a spark knock as well. EGR is disabled in park. Check your EGR valve for carbon deposits, free movement of the plunger, integrity of the diaphragm. The latter is easy to test right on the car. Pull the vacuum line off the valve, depress the diaphragm with finger all the way and plug the vacuum input with finger. If the diaphragm moves back, replace the valve.

Addalak, he has an 02 STS..

oops.. new technology...

The saddest thing in life is wasted talent

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I would say it could be the harmonic balancer also, The a/c compressor is located jusr to the front of it so it is possible.

GM FAN FOREVER

Nice, clean, luxury= fine automobile

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  • 1 month later...

Thanks for the inputs guys!

I have had a lot to do lately but I'll give you an update.

As I said before, car was knocking at hot idle.

I listened around with a stethoscope to no avail but the sound seemed to origin from the center of the car when standing in front of it.

With AC on, knock. AC off, no knock...hmm perhaps AC then?

No, because I noticed the sound with gear in drive or reverse AND AC off.

As I also mentioned before the harmonic balancer had a "wobble", the serp didn't run true so I pulled the serp and noticed that the sound was gone.

I have ordered a new harmonic balancer and hopefully it will solve my problem.

I did as Adallac suggested and marked the inner and outer segments with some paint but it doesn't seem like the balancer is slipping. It also felt firm and the rubber didn't seem damaged either.

But since the sound is gone when the serp is removed and every pulley and accessory runs true with no play and no noise, there simply isn't anything else that can make the sound.

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While the serpentine belt is off, look at everything that turns on it for a bad bearing or a loose spline on a pulley, and check the idler tension. In particular, look at the tightness of the pulley bolt on the alternator.

CTS-V_LateralGs_6-2018_tiny.jpg
-- Click Here for CaddyInfo page on "How To" Read Your OBD Codes
-- Click Here for my personal page to download my OBD code list as an Excel file, plus other Cadillac data
-- See my CaddyInfo car blogs: 2011 CTS-V, 1997 ETC
Yes, I was Jims_97_ETC before I changed cars.

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While the serpentine belt is off, look at everything that turns on it for a bad bearing or a loose spline on a pulley, and check the idler tension. In particular, look at the tightness of the pulley bolt on the alternator.

I've done that and there is absolutely no radial play whatsoever or grinding noises etc. The tensioner, servo pump, alternator and AC seems solid as a rock. No loose bolts or axial play either. Why can't I get easier problems? :)

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Its amazing how little things like a bad alternator bearing (that I had) can radiate throughout the powertrain. And you are correct, a little load makes it worse because of the increases tension on the belt turning say an AC compressor

It seems that you have localized, its someplace in the accessory area, damper, tensioners, accessories

I had my alternator rebuilt and they used a cheap chinese bearing in the front of the alternator and it had a flat spot on it, I took it back and gave them high quality AC DELCO bearings and had them swap them in (front and rear) and the noise was gone

You'll find it, it will be something subtle

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Its amazing how little things like a bad alternator bearing (that I had) can radiate throughout the powertrain. And you are correct, a little load makes it worse because of the increases tension on the belt turning say an AC compressor

It seems that you have localized, its someplace in the accessory area, damper, tensioners, accessories

I had my alternator rebuilt and they used a cheap chinese bearing in the front of the alternator and it had a flat spot on it, I took it back and gave them high quality AC DELCO bearings and had them swap them in (front and rear) and the noise was gone

You'll find it, it will be something subtle

I have had my share of worn bearings (mostly on Volvos hehe) but they seem like new. Not the slightest play or sound.

It is somehow accessory related. The only thing I really rule out is the AC-compressor because no AC = no sound, no AC + Drive selected = sound. No serp + Drive selected = no sound

The serp moves around sideways at the harmonic balancer quite much and I can swear that sometimes it looks ok.

I really hate throwing parts at it but sometimes it is the only thing to do.

To make an educated guess and afterwards scratch your head and cry "why me!" :D

I thought studying national economics was hard :P

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