Originally Posted by bobs
Sounds suspiciously like valve train noise, but we need more information... Does the noise match the road speed or then engine RPMs? Do you hear it when you start the engine? How about throttle? Do you hear the sound more when you're mashing on the throttle or at neutral or hailing throttle? How is the engine performing?
And in addition to BOBS perfect attempt to diagnose, where do you hear the noise, is it coming from the engine bay, or from behind you ?