[MPWR]

Yeah, if you're lucky it may be a failed chain tensioner- and the sound may be the timing chain slapping against the chain guide. The tensioner is easy to replace, and not terribly expensive (maybe $50). But I would avoid running the engine- it is not a healthy sound. If the timing chain is slapping around, it could do more damage- and that gets harder and more expensive to repair....

If you put your hand on the front of the engine (maybe just above the thermostat), can you feel the rattle? Be careful of the electric fan.