Light knocking sound in engine

[Patronus]

Background: My stock 3.2 engine runs exceptionally quiet and smooth. No mechanical noises.

HOWEVER.....

Lately, when the car is about 1/3 of its operating temperature, a light knocking sound comes from the engine. It sounds exactly like a loose rocker, but it starts at 3000rpm and it is quiet at 4500rpm again. When the car idles it is quiet. (Rockers make a noise at idle, right??)
The noise is not loud, you can just hear it when the car is moving.

When I accellerate it is quiet. When I decellerate it is quiet. When I am coasting, especially when I give a tiny little bit of gas, you can hear it. When the car is at operating temperature it is quieter, but not gone.

When I stand behind the engine, revving it manually, it is quiet at all RPM. I somehow can't seem to emulate the problem when I'm not driving the car.

It is RPM related, no doubt. Certainly ot a CV joint, gearbox chatter or anything like that.

I recently discovered that the alternator has a bad bearing (when I removed the fanbelt and turned the fan it went 'gorrrrrr') and I hope this is the noise that I hear. But I guess not. Why do I only hear it when I'm coasting?

Alternatively, can a loose rocker make this kind of noise at 3000+rpm? In my experience rockers only rattle at low rpm.

Then, worst case scenario, please tell me more about piston slap. Why, how, and how much.

Thanks

[surflvr911sc]

I have money on a CV joint. This is how they start and they only get worse from here.

[surflvr911sc]

After reading again, does speed matter or is it just RPM?

[Patronus]

Speed has no influence. RPM seems to be the main thing.

[n8marx]

Sorry to bring up an old thread but my car just started doing EXACTLY this 3 days ago. Exact same symptoms.

The problem occurs for me whether the car is moving or not so it is not a CV joint. If I am accelerating or decelerating or coasting the noise is not there, if I am cruising along with very little throttle input it rattles or buzzes.

Revving behind the car manually produces no noise. Any more ideas?

Patronus, what did you find your cause to be?

[Tim L]

Well for me it turned out to be the same as Patronus (bad rod bearing) hopefully you will have better luck.

Tim

[n8marx]

Bump

[david c.]

I wonder if it has something to do with the hydraulic tensioners?

[D7N7MASTER]

If you have access to a dyno you can run it, and watch and listen at different rpm's. If no dyno or too much $$$ - just take it to a smog center and for less than $50.- they'll run it for you. Good luck!!!

[n8marx]

A quick update. After more listening prodding and poking I have determined that my noise is not an engine noise at all. It seems to be a rattle coming from behind my passenger side rear seat. I can now induce the rattle at a very specific RPM. So, I think I just have something loose that rattles when the harmonics are just right. Now I have to find the annoying little thing.

[Jdub]

Engine and trans mounts 100%? Sway bar bushings in good nick? Sway bar rattling on the trans bottom under certain load/offload?

John