Thumping sound from engine compartment when roads are wet

[lothos]

Hi guys!

I'm hearing a rubbery sounding thumping noise from the front end or engine compartment when the roads are wet. It's not metallic sounding, and not a very loud or deep sound. More of a rubbery softer sounding noise. It doesn't seem to follow engine speed, it's more of a random *thump* pause, *thump thump* pause. It seems to happen at higher speeds, and only when the roads are wet. It will make the noise maybe five times, and then stop, and I won't hear it again until the next time it rains.

I've put on a new serpentine belt and it's snug. When I do oil changes I lube the ball joints and try to check things out and I haven't seen anything amiss, but I'm still a newbie. I'm not sure what this could be, any suggestions?

[BostonETC97]

From the description, it sounds like you're hearing the a/c compressor cycling. Next time it's raining and you start hearing the noise, hit the "a/c" or "off" button on the HVAC control to disable the compressor. If you no longer hear the thump, you found your culprit! Now to the significance of the thumping, I'm not sure. I'd guess its just the a/c compressor clutch slipping when it engages.

If your engine-bay setup is the same as mine, your compressor is the lowest (or close to it!) point of the serp belt's routing. While a ton of water shouldn't be getting up and into the engine bay, I'd be willing to bet some makes it onto the belt and gets slung around on a really wet day.

Also make sure all the wheel-well liners and tar-paper guards are in place on the front end. If they're missing, more water will splash into the engine compartment than necessary.

Hope that helps!

-Scott

[lothos]

Thanks for the reply and the advice. The HVAC is turned off, unfortunately. It was wet and pouring out again today. My commute is about 12 miles and about 10 miles into it I was hearing the noise again. I was driving about 60mph and the sound was faster than it was the other day when I was driving slower.

The splash guards and such look to be in the right place.

When I accelerate fast there's sometimes a noise that can best be described as a "turbocharger hiss" that was narrowed down to the belt. I replaced the belt and still hear the noise. There's also a rattling sound from the belt area when I'm parked and idling.

How do I narrow this down? Stethescope or something?

[BostonETC97]

Sounds like you've got it isolated to the belt, which is a good start. I think I'd start by checking the basic things first. Is the tensioner arm applying even pressure? Does it "bob" excessively when the car is idling, or when it's revved up? I hear you can use a length of hose as a 'hearing aide' to probe around the engine. I've never tried, but I hear it does work.

If the tensior checks out ok, start isolating each pulley along the belt path. One of them may have too much play, or have worn bearings. You'll have to remove the belt and spin each pulley (alternator, p/s pump, a/c, tensioner, etc.) by hand, and push/pull on them to see if there's excessive run-out.

The only other thing I can think of would be a failing harmonic balancer... but I'm not quite sure exactly how that'd fail, or even if it'd give you the symptoms you're experiencing. Hopefully somebody with a bit more experience than I will chime in on that!

[BostonETC97]

And I forgot to mention... When I had a bad 'rapping' sound coming from my belt, turned out the bearings in the a/c compressor were gone. Ended up replacing the compressor, and it was quiet again!

[lothos]

Hey, thanks a lot for the quick reply. You've got some good tips. The tensioner held the belt tight but I didn't see if it was bobbing. I'll check out everything you suggested and post back.

Thanks again.

[lothos]

I just got back from Dallas, and I'm in the process of moving tonight and tomorrow, and my laptop is in the shop for repairs, so I will try to check everything out and respond by this weekend.

I heard the noise one time now when the pavement was dry and I was idling the car. That was the first time I heard it when it wasn't wet out. It was really wet out this morning and it made the noise again on the freeway but stopped halfway to work, I'm assuming because whatever was wet had dried. I'll look into it ASAP so I have something concrete to report.

Thanks BostonETC97, I really appreciate the help.

[winterset]

not sure where the compressor is on your car, but a trick I was shown when I was a kid was to put a stick (like a sawed broom stick) to the componant, and the other end of the stick to your ear, and you wll near the sound as if you had your ear to the compressor (something I don't recommend). Just be careful the stick does not hit the belt, or anything spinning, as it can do some damage (especially if it's on your ear!!!) also no baggy clothes & do wear short sleeves when working with moving parts.

[BostonETC97]

I still say it's the a/c compressor, as it does it while the car is idling, and at speed. There aren't really any other components that mechanically cycle at "random" intervals regardless of engine speed or vehicle speed.

Have you added any refrigerant lately? I once made the mistake of adding too much r134 to the a/c system, which caused the compressor to "slam" when it engaged, and did feel much like a blunt rubbery "knock" from inside the car. Having my mechanic vacuum out all the refrigerant, and then re-add the proper two pounds fixed that fast!

Dallas sounds warmer than the Northeast! Why do I live here again?

[Texas Jim]

Dallas sounds warmer than the Northeast! Why do I live here again?

It was 68 today.

Supposed to be close to 80 tomorrow.

[BostonETC97]

It's been in the mid 40's and raining most of this week. And it was a LONG winter! We didn't get any 'major' snowstorms, but it seemed we got a constant 6-12 inches every week.

I think what I hate the most is how dirty everything becomes. The roads are trashed with potholes and sand. I live about 5 miles outside of Boston, but whenever I'm downtown on a windy day, it's like you're getting sandblasted from the roadsalt/sand that goes airborne in a big cloud of filth.

Spring, Summer, and Fall are really beautiful around here... but I'm only 27 and ready to move down south. I'm seriously questioning if living up here is worth the battle.

[KHE]

It's been in the mid 40's and raining most of this week. And it was a LONG winter! We didn't get any 'major' snowstorms, but it seemed we got a constant 6-12 inches every week.

I think what I hate the most is how dirty everything becomes. The roads are trashed with potholes and sand. I live about 5 miles outside of Boston, but whenever I'm downtown on a windy day, it's like you're getting sandblasted from the roadsalt/sand that goes airborne in a big cloud of filth.

Spring, Summer, and Fall are really beautiful around here... but I'm only 27 and ready to move down south. I'm seriously questioning if living up here is worth the battle.

See if you can stand the heat and humidity of the South before you decide on a move. Personally, I can't take triple digit temps. and high humidity but that's just me.

[BostonETC97]

I've got lots of family down in South Florida, and I'll admit to enjoying the hot August days. On the other hand, I don't think I'd deal with hurricanes well at all. I had to sit through a strong Cat. 2 a few years back, and if I never had to do that again, I'd be very much OK.

I suppose time will tell. The job market is bad here, but it's even worse down there I hear. I've always lived in Boston, and I'm starting to get the itch to try something new! (As long as I can still detail my caddy weekly! I'm not giving up the car to live in NYC!!!)

[lothos]

I still say it's the a/c compressor, as it does it while the car is idling, and at speed. There aren't really any other components that mechanically cycle at "random" intervals regardless of engine speed or vehicle speed.

Have you added any refrigerant lately? I once made the mistake of adding too much r134 to the a/c system, which caused the compressor to "slam" when it engaged, and did feel much like a blunt rubbery "knock" from inside the car. Having my mechanic vacuum out all the refrigerant, and then re-add the proper two pounds fixed that fast!

Dallas sounds warmer than the Northeast! Why do I live here again?

Yes I did add some refrigerant lately. That makes sense, I'll call my buddy and see if he can take some out with his machine. I still tend to only hear the banging noise when it's wet outside, but two or three times now I've heard it when it's dry outside.

I checked my tensioner, and it does not move when the car is idle. The sound I'm getting is a "moving chain" sound, like the accessory belt is a chain instead of a rubber belt. Maybe a bearing of some sort? I think I should invest in a mechanics stethoscope that I saw at Auto Zone for 9 bucks.

[Texas Jim]

I think I should invest in a mechanics stethoscope that I saw at Auto Zone for 9 bucks.

That sounds like a good idea.

[lothos]

I think I should invest in a mechanics stethoscope that I saw at Auto Zone for 9 bucks.

That sounds like a good idea.

I'll do that this weekend and post back what I figure out.

Thanks for all the help guys!