jhb

My 97 is making a strange harmonic humming noise coming from somewhere in the rear section of the car. It has been doing this since I purchased the car in August, and it is driving me crazy!

Essentially, this noise starts after I attain and maintain a speed. If I slow down or speed up, it stops, but then comes back when I start driving steady.

The frequency also does not change with the speed of the car, in other words, if I am doing 60, the harmonic is the same as if I am doing 35, so I do not beleive it is related to the engine or wheels.

It is a slow harmonc, takes about 1 second to repeat. I at first thought it might be air passing over the car and turning a fan/blower motor somewhere. Could this be the case?

I am going to have it looked into this spring, but thought I might get a jump on it with input from the board. Any suggestions. Thanks in advance!

jhb

Well, I have narrowed down this problem, Now that the warm weather has arrived I am getting to know my car better. This noise does not occur as previoulsy noted, but occurs in ANY gear at 2900 RPMs. It will even happen if I rev the engine when out of gear. I disappears when the revs go below 2800 or above 3000.

Could this be the belts noise problem I have heard mentioned, or is that a more steady, non pulsating noise?

Bins

Are you sure that its from the rear.

Sounds very much like the motors from the ventilation system in front of you. I would definitely call that a harmonic hum.

Quite nice once you get used to it.

jhb

I am sure its from the rear, and to dependent upon the engine rpms. It has to be something related to the engine, but thanks for the input.

tom_993

Since it happens when you rev the engine while out of gear, try opening the rear hood and revving the engine from there, by carefully turning the throttle. Listen to where it’s coming from.

Singh

I used to get a harmonic "resonance" at times - its no longer there. Just had a 24k service. Seems to have been solved by a combination of new rear spoiler wall (which had split, a common occurence) and two new belts.

Good luck

Ajit.

Robin 993DX

[quote]Originally posted by tom_993:
<strong>Since it happens when you rev the engine while out of gear, try opening the rear hood and revving the engine from there, by carefully turning the throttle. Listen to where it’s coming from.</strong><hr></blockquote>

Tom,

His 993 is a 1997, which means it have all the varioram piping stuffed above the engine. There are no room to access the throttle cable or throttle body from the engine bay like the 95 993s.

Bins

Can you get someone else to press the gas while youre checking things at the rear. Just make sure youre not in gear. <img src="confused.gif" border="0">

jhb

Yeah, like reverse! <img src="graemlins/burnout.gif" border="0" alt="[burnout]" />

I intend to do that this weekend, maybe I can get to the bottom of it then, I'll keep you posted.

DJ

Since you just recently got your car, and are still getting to know it, could it be possible that you have an aftermarket exhaust system? There is one system out there that is known to have a rather obnoxious harmonic resonance in the 2800-3100 rev range, but for the life of me, I can't remember the name of that exhaust.

I need a vacation.

tom_993

D.J.,
You really do need a vacation. When I bought a pair of those mufflers, the web site said “No resonance!”

Tom
’95 993

WillBank

"Fib" Fab?

jhb

OK, took a look, the sound is coming from the engine cooling shroud area. There are two belts there and the outside one (the smaller pulley) is starting to break up. I suppose it could just be the belt, but a mechanic I know mentioned it might be a bearing going bad, which is causing play in the fan, causing the hum, and the belt to wear prematurely? Any thoughts on this?

BTW, what are the inside and outside belts for? I assume the inside belt drives the fan (it has the sensor wheel on it in the lower left by the main pulley), but what is the outer belt for?

Robert Henriksen

One's for the alternator, which lives inside the fan shroud. I'd guess the alternator is the inner shafter, and the fan's driven by the outer surround, but that's just a guess. Robin knows the layout of the fan & alternator better than anyone, I think

John D II

JHB,

I would try this: My experience has been that improper belt tension, usually a little too tight, can produce a noise at certain RPM's. I would change and adjust the belts and see what effect that has on the sound. My guess is that it will disappear. Try adjusting the belt so there is moderate deflection with moderate pressure, but not loose enough to slip under full throttle acceleration.