Hyundai - Elantra :: Rumbling Sound That Corresponds To The Wheels Moving At Least About 15 - 20 Mph

I recently bought a 2009 Hyundai Elantra Touring. It only has 53,000 miles but I notice a rumbling sound that corresponds to the wheels moving at least about 15 - 20 mph. It's FWD and the front two tires were replaced recently by the previous owner. The mechanic who checked it out before I bought it thought that it was the rear bearings, but when he looked at the tires, he was pretty sure the sound was coming from the tires. I don't mind the noise, but if it's not the tires, it's something I should fix.

I took it to a shop today and the mechanic today and he drove around with me and though the sound was coming from the front of the car and thought it was either something with the rotors or the bearings. I've gotten the speed upwards of 70mph on the highway and I wouldn't say the sound gets any worse when you do that. (I mean you can still hear it, but for instance I didn't even notice it on my first test drive, and the radio mostly drowns it out.) Also given that these two mechanics can't seem to tell whether the sound is from the front or the back, I'm not sure that rotating the tires and listening is going to be useful as a lay person.

It's not a "clunk da clunk" sound, rather it's like a vacuum cleaner hum. Also maybe relevant, the rotors and pads on the rear wheels need to be replaced pretty soon.