Tonight I noticed that there is a strange sound coming from the rear of the car. I can only hear it with the window down and it seems to be coming from somewhere around the back tire but it's hard for me to tell while driving. The sound is variable with speed and sounds like it could be something with the tire but I haven't found anything yet. The best way I can describe the sound is if you took a rubber flip-flop style shoe and were tapping the sole on the ground over and over and increasing in frequency with speed. Any easy answers for what this could be?