My Car Goes "ping"

This had been going on for a long time. Does not seem to affect anything im just curious as to what it is. About 80% of the time I let off the gas I hear a little "ping". I was wondering if it was my TB closing or something...im not sure. It seems to be louder if i only give it a little gas then take my foot off. Any ideas? Its not loud or anything and you have to be looking for it it hear it most of the time.