Sound from rear of car

I 've had my 2011 for nearly 3 years and I know this sound is normal, but never really cared what it was until now. Usually within the first few minutes of my morning drive, there is a sound, sometimes two or three times which sounds as if you ran over a piece of rubber and it hits the wheel well. I always figured it may be something to do with the gas tank pressurization. Kind of like a low pitched thump or thud type noise. Pretty distinct noise, I'm sure any gen 1 owner will know the sound.
Anyway, what is it for sure???