A year or so ago, I noticed a faint noise while driving at certain speeds. It sounded like it was coming from one of the rear tires, but was hard to tell, and the noise could only be heard with the windows closed. It'* not a squeel or whistle or hum; it'* a faint low-pitched "m-m-m-m" that increases and decreases with speed, but can only be heard at a specific speed range from maybe 25-45 MPH or so. Because 2 of the 4 tires were close to needing replacement at that time, I took it to the tire shop, expecting to buy 2 new tires. They said the 2 older tires were making the noise. So I had the 2 older tires replaced. The noise was still present. I drove it for a few more months and then took it back to the tire shop and reported the same noise again. This time, they said my shocks and struts were shot, and caused scalloping on the tires (the new ones) and now the new tires were making noise. They installed new shocks and struts. To my surprise, the noise remained, so I gave up and reluctantly learned to live with the noise. Last weekend I took it in again and told the shop about the longstanding noise problem. This time, they offered to sell me 4 new tires with a small discount. I bought 4 new tires thinking the noise would be eliminated. I drove the car to work today, and to my surprise, the same noise is still present, even with the new tires and 4-wheel alignment!

What could cause a noise like this, a noise that varies with speed but supposedly isn't brakes or wheel bearings and can't be tires? The first time I took it in and reported the noise, I also asked the shop to check the brakes, wheel bearings, and other possibilities. They said everything was fine; the tires were making noise.

Bill...seems like they've harvested a nice chunk of cabbage from your wallet.

Is there anything loose that could be getting caught in the breeze at that speed?
Splash shield
Cladding
Vent Visors

Nope, I didn't see anything that appears to be loose, but because the noise varies with speed, I suspect that the cause must be something that'* rotating.

I just called the tire shop and told them that the noise is still there.. They apologized for the misdiagnosis and suggested for me to take it to another shop or the dealer...so far, I've spent roughly $1,1150 to get rid of the noise! I'm not sure what to do next.

Sounds every bit like a wheel bearing on its way out. Has the noise gotten any louder since you first heard it? For a tire to make the kind of noise you describe from cupping, the cupping would have to be pretty severe and would be easily visible. I got the "weak suspension makes your tires cup" speech once when I let a pair sit on the back too long (they needed replaced anyway) and I laughed at it then as I do now. Not rotating your tires causes them to cup. Weak suspension plays a negligible role.

My money would be on a wheel bearing. Is it obviously coming from one side more than another?

Bill...if possible have other friends (mechanically inclined preferred) to ride with you. Sometimes it'* the buddy that hears it.

The first mechanic who investigated said if it were a wheel bearing, the noise would increase in frequency and get louder with speed and would be more of a constant squealing noise. It sounds to me like the noise is coming from the left rear, but it'* hard to tell because the noise is so faint. Different people have listened, and they've thought it was coming from different wheels. As far as I can tell, the noise hasn't gotten worse or changed symptoms at all over the last year, and I drive roughly 900 miles a month. For what it'* worth, when I first noticed the noise, I suspected one of the rear drums.

You know, just because it changes at different speeds and only is noticable at a certain speed range, does not mean that it has to be something that is rotating. First, there is the typical poor noise reduction insulation in the rear of our cars. Mine will hum at certain speeds. Always has, probably always will. There is something called harmonic vibration that causes this phenomenon, typically caused by a shortcut during noise attenuation design studies (otherwise known as budget restraints).

This is not to say that you don't have something wrong, just in case you do. However, you say that you just noticed this noise a year or so ago. Is it possible that it was there before you noticed it?

If you've already eliminated all the typical mechanical reasons for this hum, it may be you are chasing an expensive ghost problem. Just another possible take on it. Don't hesitate to prove me wrong... I wouldn't want to be responsible for you ignoring a real mechanical issue...

The noise not changing is why I asked about wind noise. Having gone through tires, a wheel bearing should have gotten worse.
Edit: Hmmm maybe pulling the back seat for a couple days would give you some insight. When I pulled mine..everything in the back was much easier to hear. (like my failing fuel pump)

What it sounds like will depend on how the bearing is failing. The last time I had one go, it was a low humming sound that increased in volume as speed increased. There was no squealing, even when the bearing had close to an inch of play from side to side. However, mine did get louder over time as the bearing got worse, which it doesn't sound like yours is doing.

The noise was not always present; unfortunately for me, I'm the type of person who notices noises.
Is your "humming" noise a constant hum when it'* happening? Mine isn't - kinda pulsates when it'* happening, and no, there doesn't appear to be any mechanical vibration or motion felt when the noise occurs.