vibrating sound from the rear wheels area
 

I just purchased a used 2003 CLK320 with 30k miles. After a day of driving I noticed a strange vibrating sound from the rear wheels area. It seams independent of the speed, acceleration, breaking or turning. I tried turning off ESP with no effect. Randomly maybe for 4 to 6 sec, maybe once every 5-10 min I hear a vibrating sound from the rear wheel area. It sounds exactly like the sound the tires make when driving on ribbed surface of the road. In fact that’s what I thought it was when I first heard it. Its not metallic but like the tire vibration when approaching the tollway boots on a tollway.

Any ideas what this could be?

I know its a bit hard to deal with noises etc threw a forum but perhaps someone had exact same symptoms.

Thanks
Darius