Engine Grinding Sound - 91 300E
 

Actually a couple strange sounds have occurred, maybe they're not related.

First, I started hearing a loud howling/whistling sound. It was extremely loud and would occur between 1750 and 2000 RPM. Very strange. Looked under the hood, nothing is out of place. That stopped after about a day.

Secondly, just last night I started getting what can only be described as a loud "grinding" noise from the engine area. It is definitely RPM dependent and is most noticeable with the windows down next to a curb. Sounds like a metallic scraping noise which increases as the engine is revved. I haven't looked under the hood yet; perhaps someone has suggestions as to what to look for.

I don't suspect the engine - it's in top shape. And I know a grinding noise like this would kill the engine in about 5 seconds.

Belt tensioner and water pump are new. I don't think A/C clutch is the culprit, as the noise doesn't go away with the compressor engaged. What else can make such a loud scraping metal noise? Alternator? Air pump (does my car even have one)? Transmission shaft? The noise occurs whether in P, N, or D.

Any ideas anyone?

Thanks!

Air pump, yes you have one. It is located above the alternator on your left hand side if you are standing in front of the car looking into the engine compartment.

This happened to me one morning while on an out of state trip. Pulled into a gas station after a 4 hour interstate run. Filled up. Upon start up a horrible screeching grinding noise began. It was easy to diagnose. Simply take a long handled screw driver and grip it with your thumb over the handle end.
Place your bent thumb joint to your ear and touch the blade to the air pump.
You will hear the noise if that's the problem. You can also pickup a cheap stethoscope at MacParts.

The good news is that the air pump is designed to only run on cold start up.
Trace the wire to the connector and disconnect. That disables the clutch and the pump should not turn.

Replacement rebuilt pumps are 500 bucks ish.

Steve

Alternator is a good guess, or fan bearing (if clutched), but I would remove the belt and see how all the bearings feel by hand. With a new tensioner, the added tension you likely have now could have pushed a marginal bearing over the edge.

But try first: if you manually clutch the fan IN by shorting the switch contacts, the sound will vanish if the fan bearing is the source.

Steve

When I say "new" I mean within the last year and a half - not brand new. And I know the belt is not too tight - in fact, it's slightly loose because if I accelerate very hard the belt will slip and kill the A/C.

bump - anyone???

it is a long shot
 

but i suspect the idler pully. i know you say it is new but it still could be the problem.

tom w

How about the starter bendix not disengaging.

John Roncallo