05E60Black545i

I just noticed it the other day, I drove about 40 miles and when I got out real quick to run in to get something, I came out and heard this faint "thumping noise"-thats the best i can describe it as- but when you are at the nose of the car you can't hear it, it's only when u are at the sides by the front tires. It only seems to occur if you take the car for a longer drive, because if you start the car when it's cold it does not do it. Any ideas what this could be, because it doesn't sound like it's valve lifters, because I can hear those when it's cold, this is a steady thumping noise and it's rpm dependent.