[abhattan]

If the sound is low pitched and slow, it is a rod knock. It will go away as the rpm's increase. If it is higher pitched (not too high) sound, then it is valve noise. The rod noise will also be a little slower than a valve noise because the bottom end of the motor moves slower than the top end. If it is in between and sounds more like a rattle, then it probably is the wrist pin. This also goes away to a degree with incresed RPM's. However, you will hear it most upon acceleration and deceleration. I believe it is either a rod knock or a wrist pin knock. These are the things that could go wrong in your cenario.