strange sound?

[Snibble]

I don't know what the sound is from.. I hope its not something big. Anyways... to get that sound, I would just accelarate normally and right when I feel the car is about to shift gears, I take my foot off the accelarator and a sec later I hear a "clunk". Is this just the gear shifting back down? or is this something bad? I get that often in city driving due to stop lights and cars infront of me... so... I just want to know what that could be.