Intermittent Engine knock, pulley?

Hey guys, 2009 jetta tdi. 325k km. DSG

While at idle I have a knocking noise coming from the serpentine belt side of the engine.

If I apply a little gas the noise usually vanishes totally. It's not a consistent knock but comes randomly, not synced to the rpm. It sound like a fingernail tapping Morse code on a can of cat food.

My alternator pulley was seized, the belt bounced a lot. I thought I had found the culprit but after replacing the alternator pulley today I still hear the noise. (maybe not as often) but the belt is much smoother, could it possibly be a bad tensioner pulley bearing?

Anyone have any similar experience?

Definitely could be a bad tensioner. Having a bad alt pulley puts a lot of stress on the drive belt system, if you take the belt off and run the car do you hear the noise?


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kdawg89 said:

Definitely could be a bad tensioner. Having a bad alt pulley puts a lot of stress on the drive belt system, if you take the belt off and run the car do you hear the noise?


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I agree, I replaced 2 on my Mk5 in 474000 miles. You can normally see it bouncing around once you take off the engine cover off.


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Thanks for the input,

I replaced the clutched alternator pulley today , but the noise still remains. I will try running without the belt tomorrow but as the noise isnt always there it might not be very conclusive.

I am saying the bad alt pulley May have ruined your belt tensioner and possibly the belt tensioner is making the noise. Generally speaking drive belt systems are designed with roughly 100,000 mile life on the belt and pulleys and tensioner. Same concept on why you never replace just a timing belt, the bearings are worn out and more prone to failure after that long.


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Also, check to make sure the harmonic balancer bolts are tight.

In 2010 or so, my 2006 BRM engine began making similar sounding noises during idling. The problem turned out to be DMF failure, which at low RPM threw the rotational balance out enough to move the engine just enough to bang an exhaust shield against something. There were actually a variety of odd sounds being made, but the most unexpected noise sounded to me like what you first described.

I'm having the same problem as the original post. I'm due for a timing belt and have already receive timing belt kit from idparts. Since I'm replacing the timing belt I figure it would be a good time to change the alternator.

The question for group: Is there any particular brand of alternator would you recommend over the others?


Or would you just replace the alternator pulley?

I would just replace the alternator pulley. The alternator is a very easy to r&r down the road. A Bosch re manufactured alternator would be the way to go if you do replace it and don't want to mess with it again. For my truck, reman Autozone alternators(super easy to replace and I'm not left on the side of the road if it fails) are good enough, but I wouldn't do the same for the car given the electrical headache.

Just replaced the alternator pulley on 09 Jetta tdi, the original pulley was seize up. The belt now runs smoothly.

Thanks crazyrunner33 for your input.


Guess next weekend I'll attempt replacing... the timing belt