Unknown sound problem

I own a 2000 Mustang (automatic) I recently purchased used from a dealership.

A few days after I purchased it I noticed this sound. Here's the best way I can describe it. At speeds of about 30 and below it's a deep wobbling sound. It's like the slow wobbling hum you get if you wet your finger and run it around the rim of a glass, but much deeper. As I accelerate, the frequency also increases. At faster speeds it sound the same as if I was driving with a flat tire.

There is no vibration when I drive at any speed. I'm confident the sound is coming from the driver's side, and fairly sure it is coming from the back tire on that side.

The tires are fairly bald and I'm not sure if there are any bulges but I would expect there to be some vibration from that. It needs a brake job as well, but other than there doesn't seem to be any other obvious problems.

Any ideas?

could be the wheel is not balanced properly, are you sure its not drivetrain related? the gears in the rear will howl when they are going bad.

check out you driveshaft . one of the counter weights could have come off . they are tach welded on the front or rear of the drive shaft

Tire being bald could be the problem. I've seen it happen like that before. The way you described it sounds to me like the left rear wheel bearing is shot. They moan deeply when going bad and it goes with the rpms of the car. Jack up the rear and see if you hear anything while rotating the wheel. There also should be no play side to side in the wheel or the bearing is bad. Or pull the wheel and make sure the brake rotors aren't rusted too. They can give that sound when rust accumulates on them because the rear calipers arent moving freely. If you have drums on the rear then it's probably the bearing. Hope this helps you.