I drive a 1999 Toyota Camry, which usually runs very, very quietly. Lately, however, I’ve started to notice a humming noise from the back of the car while I’m driving.
The best way I can describe the noise is that it sounds as if there is someone in the distance driving a vehicle with a loud engine, like a large truck or a motorcycle. It’s not a very loud noise–in fact, someone who didn’t ride in the car regularly might not even notice it. But it does get louder as I accelerate.
I don’t feel a “pull” while I’m driving, as if the transmission were sticking in a lower gear. But it does sound like something in the rear of the car is having to work harder than it should.
Assuming that I am not always being followed by someone driving a loud truck or motorcycle, what could it be?
Step 1 for me might be to rotate the tires and see if the hum moves. Take a hard look at the tires while you are at it.
If you here the hum, but not moving, I would suspect the fuel pump. If the hum starts when you are moving, and changes tone as you speed up or slow down, I would suspect a wheel bearing. As suggested, rotate the rear tires and hear if the hum moves. If not, have a mechanic check the bearings.
You might also have an exhaust leak.
BustedKnuckles is correct – almost 100% a faulty rear wheel bearing. Any mechanic can isolate this on the rack and replace the bearing - not a big deal and not terribly expensive to fix.
Thanks guys. I’m taking it in on Friday–I’ll let you know what they find.
I agree with the others - sounds like a bearing…
Or a VERY annoying passenger.
The latter is easier to fix.
Well, basically everyone was right. 
It was the left rear wheel bearing. But I have more questions, which I’ve posted as a separate discussion, http://community.cartalk.com/posts/list/824209.page