Stan, your sense is if the noise is coming from the air pump, then you wouldn't be too concerned about the fore & aft movement of the belt? Or are you suggesting that I first test the air pump to eliminate it as the culprit - going on the assumption that I'll be pulling the cover and checking the t/b system anyway?
Because as soon as I looked down onto that pass. side gear and saw the glimmer of shine coming from the teeth, I realized that something wasn't right with how the belt was tracking .
I was holding out hope that a belt in need of a periodic tensioning would cause it to wander a bit, but I've gotten no belt tension warning, and a belt would have to be pretty loose to walk like that.
Oh well.
Andrew, thanks for the link. That's perfect. I'll use it the next time that I'm simply going in for a tension check (which probably won't be this time).

 

Do you actually see the belt tracking unevenly (moving fore and aft) if you peer down the inspection holes with the motor running? What's the frequency off the noise at idle?

 

What Bill said that was my next question as well

please run the car and watch the belt through the holes report back

 

Today I started the car from cold and listened for the sound that I describe in this thread. Couldn't hear it. I have to assume that it comes about when the engine is hot.
I borrowed my daughter's Flip video (how handy are these things?!) and recorded the runout of the belt on both cam gears - first the driver's side; then the passenger. Based on the runout that's seen in the video, I think that the pictures that I took captured the extremes on both sides.
The fact that the noise didn't immediately come about combined with the moderate runout has me in a bit less of a panic.
What are your opinions of whether this amount is acceptable & what would cause this fore/aft runout?
Anyway, here's how it looks:

http://www.youtube.com/watch?v=ApqoI45ec20

 

With how little work it is to pop the covers off and inspect everything, you should really do this post haste.

 

Yea, I agree. I need to get in there for a retension at the very least. I just don't have the time to get in there right now. My curiousity is killing me, though.

 

I've seen some mention of some belts having a bias that can cause them to have some runout but I have not seen it. Maybe you just have a crooked belt. That is a small amount of runout, but just the same, the back edge of the belt may rub the tensioner arm as it moves back. I've seen them be very close to doing so. Did the noise match up the the movement of the belt?

Also, I've never understood why Porsche states the belt should be installed with the print readable from the front, at least starting in 87. If the belt is somehow directional why not put a directional arrow on it? Also, I've never seen one with a lime stripe.

 

Weird.

I don't even know what to think. Get the covers off and let us know what you find.

 

IMO the runout on the gears is OK and about what I would expect. What is not normal is the runout on the belt. Something in the belt system is leading to the belt wobble.

I'd go ahead at let the engine warm-up and see if the noise can be better-localized. And I'd pull the side belt covers and have a look-see. You can at least check the condition of the idler roller, belt tension, and the location of the pump pulley. You can also loosen the center belt cover so that you might be able to get a peek at the crank gear and main roller.

Look for one or more of: pulley migrating, roller carrier bushings not quite right, carrier brace missing, one or both big "washers" on the crank gear missing, idler and main rollers not aligned with the belt and/or each other. Look for belt dust too.

 

FWIW, I had a very similar sound on my 86,only when cold, noise matched the belt movement. Turned out to be the boot on the end of the tensioner was getting snagged by the belt as it tracked back and forth. Lasted for about 2 or 3 miuntes, then went away and only came back when completly cold. Removed the covers, turned the engine over slowly and saw it getting snagged.
Rebuilt the tensioner, new bushes on the arm, and new belt - now quiet as a mouse !

 

..so a bit of an update:
I haven't gotten a Kempf tool as of yet, so I've delayed removing the covers until I have all of the right tools.
I know....asking these questions without pulling the covers doesn't help to diagnose the issue, but the input that I've gotten thus far has helped me prepare for what it likely is & what it likely isn't.

Figuring that the belt condition, tension & tracking isn't in need of absolute urgent attention, I've driven around a bit.
The sound isn't noticeable at startup, when cold or warm.
It's when the engine is hot that it becomes noticeable.
It's sort of a sewing machine sound, and although it does somewhat emanate from boths sides of the cam covers, it's more noticeable on the passenger side.
Using the steth, it's loudest on the cover itself than on the distributor or air pump bracket.
I haven't checked the runout when warm vs. cold, which is something that I should do. If I were to guess, I'd say that it gets worse when hot...but that's a guess.
So I'm thinking tensioner roller or tensioner arm. The sound is from that general area.
Tell me if this make sense:
As the engine warms, the tensioner applies more tension to the belt via the arm & roller. If there's an issue with the arm or roller (bearing for instance) it would emanate across the tight belt, and wouldn't it become more pronounced with a warm engine?

 

Sewing machine? Could it be the injectors?

Dan
'91 928GT S/C 475hp/460lb.ft

 

It sounds a bit like injectors. That's what I checked initially, but the steth tells me it's not coming from there. It's emanating from the belt covers (mostly the pass side). I'm guessing the covers are relaying a much louder sound coming from some component of the t/b system...but again, that's a guess.

 

Could the tension be too tight if the noise becomes more pronounced as the engine warms up?

 

JP,

I have a Kempf tool you can borrow.
.