Clunk, clunk, bad news?

[dammad]

I noticed that my 99 996 cab has seem to become a little noisier to drive recently. More 'gruff' in engine note, but otherwise runs fine.

Out of curiosity, I parked and left it ticking over to check out the engine. On opening the engine compartment, I noticed immediately that there was a noticeable (but not from inside the car) 'clunk, clunk, clunk' noise coming from the top center of the engine with a period of about 2 secs for 3-4 clunks. I turned off the AC, fan, etc. to help rule out those, but it remained. Alas I was on my own so there was no way to increase the RPM while listening but will try that when I get home.

I've also noticed that the AC causes a bit of noise when on (audible from inside the car after running it for a while). Do you think the noise might be the bearing going on the AC or something worse (blow engine)?

Given the speed/idle it seems like it could be the tappet on one of the cylinders, but why would that 'clunk' rather than tap as normal?

The car has 105k miles on it and runs well otherwise. New mobil 1 0w40 about 1000 miles ago. O2/MAt done about 10000miles ago. No CELs or anything.

??

Dammad.

[1schoir]

I noticed that my 99 996 cab has seem to become a little noisier to drive recently. More 'gruff' in engine note, but otherwise runs fine.

Out of curiosity, I parked and left it ticking over to check out the engine. On opening the engine compartment, I noticed immediately that there was a noticeable (but not from inside the car) 'clunk, clunk, clunk' noise coming from the top center of the engine with a period of about 2 secs for 3-4 clunks. I turned off the AC, fan, etc. to help rule out those, but it remained. Alas I was on my own so there was no way to increase the RPM while listening but will try that when I get home.

I've also noticed that the AC causes a bit of noise when on (audible from inside the car after running it for a while). Do you think the noise might be the bearing going on the AC or something worse (blow engine)?

Given the speed/idle it seems like it could be the tappet on one of the cylinders, but why would that 'clunk' rather than tap as normal?

The car has 105k miles on it and runs well otherwise. New mobil 1 0w40 about 1000 miles ago. O2/MAt done about 10000miles ago. No CELs or anything.

??

Dammad.

If you remove the serpentine belt and run the engine for 30 seconds, that should help you eliminate, or, better yet, identify one of the pulleys (or their bearing(s)) as the culprit.

Regards, Maurice.

[dammad]

Thanks.. Didn't know that was possible.

I let it to cool down and started again. This time the clunk, clunk is more of a whurr, swish, swish noise now with the same kind of speed. So it sounds like an alternator or AC pump (water pump was changed at 90k).

Edited February 11, 2009 by dammad

[1schoir]

Thanks.. Didn't know that was possible.

I let it to cool down and started again. This time the clunk, clunk is more of a whurr, swish, swish noise now with the same kind of speed. So it sounds like an alternator or AC pump (water pump was changed at 90k).

If it is the alternator or the AC pump, running the engine for a few seconds with the serpentine belt off will give you an immediate answer (and hopefully some relief from any anxiety about a more major problem).

Regards, Maurice.

[dammad]

Decided that I might be able to work out the problem just by taking off the airbox and listening (down a pipe) to localize. Seemed a little less ambitious.

It seems that the noise is coming from the top roller on the serpentine belt by the alternator. Are these known to go bad? I assume they have some bearings inside. Is there anything near that roller that it could be instead?

[Loren]

Decided that I might be able to work out the problem just by taking off the airbox and listening (down a pipe) to localize. Seemed a little less ambitious.

It seems that the noise is coming from the top roller on the serpentine belt by the alternator. Are these known to go bad? I assume they have some bearings inside. Is there anything near that roller that it could be instead?

Yes, the pulleys go bad.

If you let it go until it breaks - it will sometimes take out the alternator too. I suggest you replace it asap.