Even mechanics need a little advice now and then... odd noise

wastintime · 03-04-2008, 05:25 AM

Hey everyone,
I guess I'd be more worried if my car was running badly, but it's running so well that it's hard to really freak out.

Basically, my car spent a couple of months without turning over because one of my best friends is useless. Then I took the car on a 600 mile drive. The only real problem on the drive was the realization I needed to replace the tires soon, lol, now I need to replace them like last week.
On the last leg of the trip I noticed a slight noise coming from the engine compartment that seemed to resemble an audible vacuum leak. At first I thought a vacuum line that I had plugged had just either broken or come unplugged, but no, that's not the case
I heard it occaisionally, finally last night it did it as soon as i cranked the car, so I hopped out and opened the engine cover to listen. With the engine cover open it's actually kind of a frightening noise... It was loud, and of all odd places seemed to be coming from the fan or the alernator maybe? The alternator is literally brand new and working like a champ. The fan looks fine when it's stopped, no nicks or anything like it was hitting the shroud.
I wonder if one of the bearings on either the fan or alternator is dmaged, but again, it doesn't sound like a spun bearing or anything like that. It sounds exactly like a vacuum leak, it's high pitched and sounds like air getting sucked through a small opening. It's intermittent, and then goes away. Never know when you're going to hear it again.
Eh, I don't know... maybe it's got to do with the cold weather here that the car has never seen before? I'll keep investigating, but just thought I'd see if anyone had a similiar experience or had any ideas that immediately jumped out.

Thanks,

Grady Clay · 03-04-2008, 05:41 AM

Andrew,

The alternator bearing can give an intermittent ‘chirp’ sound. It is also possible that the outer pulley half(es) are not on straight and the noise is the pulley part wobbling.

Does your RSA have the two speeds for the fan/alternator? Could it be in the bearing between the two?

An easy test is to remove the belt(s) and see if the noise continues.

A stethoscope with an open hose can occasionally locate the source of the noise.

Best,
Grady

wastintime · 03-04-2008, 05:50 AM

Grady, I was thinking about a variable speed thing, I'm honestly not sure on that alternator, I'll look it up in my 964 book.

Lol, I've got a stethescope, in fact, I just posted on how useful they are somewhere... that just confused me more... that leads me away from the fan. I had thought about the belts themselves, but they're new, their tension feels fine too. The only reason I'd like to think it isn't a bearing in the fan/alternator assembly is that they are all brand new... less than 1 yr old. Which is not to say a new part wouldn't be more prone to failure than the original from '93, but it seems unlikely.

The real trouble is that if I start the car and it makes the noise, there is no gaurantee that when I turn the motor off and take the belts off and crank it again that it wouldn't have made the noise anyway... it's just so random when it'll actually decide to make noise. Eh, I'll keep investigating and see what I find.

Fast Corners · 03-04-2008, 08:59 AM

My 964 had an interesting noise comming from that area as well. It has been 8 years or so since that problem reared it's ugly head so I don't exactly remember what it sounded like but I do remember that it was an odd noise. Come to think of it I has two problems in that general area.

The first was the belt tension/sender thing. The little plastic wheel was making an odd noise. I removed it spun it with my finger and since it made a noise I replaced it...noise gone.

The other noise came from a broken mounting stud in the pully system they are small M6's I believe. One of the studs was broken so the pully had a slight wobble causing an odd whirring.

The system is a lot more involved than on earlier cars and since the fan and alternator are designed to turn at different rates there may be more digging to get to the source of the problem. Atleast it's in an area that is pretty easy to get to.

FC

dshepp806 · dshepp806's Garage

so, the bearings in the alt are new?

911quest · 03-04-2008, 06:02 PM

964 have a impellar that runs off the back of the ALT. is it possible something got back there and is scrapping paper etc.

Wayne 962 · Wayne 962's Garage

You really can't hear vacuum leaks all that well on a 911 engine, it's probably something else...

-Wayne

wastintime · 03-11-2008, 06:19 PM

I'll check the tensioner.... that's not a bad idea... the crazy thing is, I swear, it hasn't made the noise once since I posted this, and that's why it's so damn hard to diagnose... It'll do it for 20 seconds at 2am on an ice cream run... when I'm in shorts and sandals without a tool around...

I did check the alternator though, A yes, the alternator is brand new, all new bearings, everything... and B it's recharging like a champ, so I'd like to believe it isn't that, but.... i'm sure it's possible. I don't know... I'm not going to freak out yet.

BTW, wayne, I know you can't hear a vacuum leak on a 911 usually, that's just the best way I can describe it... it sounds exactly like a loud vacuum leak

wastintime · 03-30-2008, 06:16 AM

Okay, so the only things I can say about the noise are it's random... never know when it'll happen and it only seems to have ever occured when the temperature is below freezing... as I said, it's weird

RoninLB · 03-31-2008, 10:03 AM

probably no help but this is what I use for vacuum hard to find leaks.

oly · oly's Garage

wastintime,

Did you find the source of this noise?

I'm having a similar problem on my Carrera 3.2. I don't have a sound insulation blanket and it was quite disturbing to hear while driving.

DRACO A5OG · DRACO A5OG's Garage

+1 on taking the belts off to attempt to duplicate the noise.

I found my bad AC Compressor bearing that way.

wastintime · 05-05-2009, 03:52 PM

oly,
I doubt my solution would help you since you've got a 3.2, but it ended up being my alternator belt specifically. I'm not sure if it was on too tight or too loose when the new alternator was installed, but it had worn very strangely... I'd imagine seeing below freezing temps for the first time in it's life helped too... the belt would wear/slip a little against the 3.6l belt sensor.

oly · oly's Garage

Thanks Andrew.

My alternator belt seems to match the somewhat vague Bentley specification of 0.2 in deflection with thumb pressure. The A/C is definitely looser but I can't find a specification for it so I don't know if its good or not. So I'll probably just tighten the A/C belt and see how it goes.