I'm trying to see if someone can give me an idea of what may be wrong, but this morning my car starting making this odd somewhat high pitched hum. It almost sounds similar to the sound you hear when you are on a subway. It happens only at low speeds for instance, this morning I was in traffic so it was kind of stop and go. When I would step on the gas, the hum would start and get increasingly more audible until the sound of the engine kind of drowned it out andfaded away.Conversely, when the car was just rolling without touching the gas, the sound wasn't audible.

Now the sound was less prevalent as the traffic broke and I began travelling at highway speeds. It's really most audible when I was driving in traffic and in the city/neighborhood. Anyone have any ideas? I scheduled a service appointment at my dealer but the earliest I could get in is on Monday the 22nd, so I'm wondering if this is sounds like something that could wait that long or if I need to be worried and see if I can get in somewhere sooner.

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Sounds like a turbo thing!

I had a similar experience with a Mercedes turbo diesel, but it never amounted to anything.

 

You would have to listen to the pitch of the sound to determine if it's a (non-problematic) turbo squeal. I had a 2003 1.8T, and the turbo squeal would start to kick in at about 2500rpm (but it would be very faint.) As your engine revs up the pitch and the intensity should go up as well. The turbo squeal will be much more pronounced if the engine is accelerating under load (as opposed to reving it in neutral.) In my '03, you couldn't hear the turbo with the windows up unless you knew what to listen for. I.E. I could hear it but it was faint enough most non-gearhead passengers couldn't. I just described the normal turbo sound, this would be expected to be present consistently--although slight changes in driving style can affect the sound of the turbo.

If the sound is abnormal there are a number of things that it could be. Since you said it was similar to a common subway sound I would first look at the alternator or a bushing/bearing.

An electrical whirring is often heard on subways due to the large motors. The alternator will make a similar sound when it's tolerances are out of whack. -- Brushes wearing out or the wraps in the coil are coming loose.

It is a mechanical squeal rather than an electrical one, it may be a bearing that is wearing.

Since you said it went away when coasting, I would say that points to a component on the engine. -- As opposed to somewhere else on the drive train.

Based on the general description you gave I would think you will be fine waiting until your service appointment. Many of the components start making such noises quite a while before they fail. However, it's always a good idea to be on the cautious side. If it were me, I would drive as little as possible until it's checked out.

 

So while I was driving home from work today I kept the music off and the windows up so I could really listen to the noise. Problem is, it didn't happen. That is, until about 30-45 mins in when I started hearing it very faintly. Now upon a second listen I gotta say, the sound I'm hearing is somewhat of a musical type of tone. In comparison, the turbo sounds more like a whistle when it engages, this sounds closer to a keynote on an electric piano albiet (sp?) a note on the low end of the scale.

So I took a different route home, one that would require a little more city driving rather than highway driving. Once again, to reiterate, the noise doesn't exist at freeway speeds. So what I noticed is that at a stop light or stop sign, when I begin to accellerate the noise is not audible. When I get to about 10-15 mph through 20-25 mph, is when the sound is most audible and if I give it gas slowly in that range, it gets a little louder but completely disappears once out of that mph range. To make things more interesting, if I step on it, i don't hear the noise.

Interesting situation huh? I'm hoping I can just wait until Monday to get it looked at but I'm a little nervous because I have such a long commute.

 

So believe it or not, I've got more to add. On my way in to work this morning, the suspect noise didn't happen at all. I made the entire 45 mile trip without a peep. But, I stopped at 7Eleven to grab myself an energy drink and when I got back in the car, it made the noise as I turned the key and the engine cranked. The noise was somewhat abrubt lasting maybe a second or so. As I was driving through town, it was back to the same conditions I've already described.

I was telling a co-worker about the problem and he had a suggestion that maybe it could be the sound of the belts. He said that it might be something that could be happening because of the fact we've had so much wet weather here lately. Could this be a reasonable explanation? I wish I could record the sound as I know when I take it to the shop on Monday that I won't be able to replicate the sound and the tech's will look at me like I'm crazy. If anyone could throw out some ideas as to what it could be that would be great so I can just tell the mechanic to take a look at x,y, or z if I can't duplicate the sound I'm hearing in his presence.