Rotational Scraping Sound
 

There is a nearly imperceptible rotational scraping sound coming from the driver's side front wheel well. I have noticed it twice, both times only with the windows all the way up and the roof closed (because its winter time, I've had the windows open for months now and never noticed it before). The sound is always present, at slow speeds, high speeds, but isn't tied with the rotation of the tire, nor anything to do with brakes, clutch, or steering so far as I can tell (the sound continues even when in neutral and rolling). It sounds as though a bit of uneven tire is snagging on a bit of fairing, or rubbing the inside of the tire-well itself.

Here's the catch, the sound is not tied to the rotation of the wheel. The wheel to sound ratio is about 1 to 4, so its much slower than the wheel rotation. The other thing to note about the rotation is that it is regular, but not even. It has a bit of a waltz feel, 1 is accented, and each successive scraping sound is less than the previous in groups of 3.

I have taken a light to the undercarriage and I can see nothing loose that would appear to be scraping externally. I would have though some irregularly shaped debris got stuck in the tread of the tire, but there is nothing loose or sticking out from anything so far as I can tell.

Do you have any tips for how to hunt down this pesky sound? Is this an indicator of something more serious?

What year? Early cars and 924S have the speedo cable running through the front left hub.

John

1986

Hmmm...have you checked your front wheel bearings? I could imagine that the bearing cages themselves might rotate between their inner and outer races at slightly different rates relative to actual wheel spin...and that if these bearings/races had started to become compromised in some way (scoring, spalling, etc.) - you might hear the result of this as being similar to what you've described.

At any rate...its a very easy matter to lift the front of the car, pop off the bearing dust cover, unscrew the 6mm allen bolt which locks the bearing/hub retaining nut - and get up close and personal with your bearings. Just a thought!

Brake dust shield got bent and is scraping on the rotor?

I'd go with both since your going to have to pull the front wheels anyway. If the bearings are shot the sound might be irregular as described.