My 01 Altima has had this metalic rattle sound for a while. The noise seems to be coming from the crankshaft pulley area. I removed the passenger front wheel and lower fender shroud, then I reomved the belt from the lower idler pulley, hoping that pulley was making the noise. I then started the motor with the belt off and the metalic rattle is still there. It seems to be coming from the crankshaft pulley, I put a screwdriver on the upper idler pulley and didnt hear anything strange. Any ideas on what could make this metalic rattle? It only happens at rpm's under 2k and goes away at higher rpm's. I did try to listen al over under the car while the motor was running and couldn't find any loose bolts or shields. I'm stumped on this one??

I had a similar rattle on my 98. It was a loose flange that holds the exhaust pipe to the manifold near the right front lower motor area. I swore it was from the motor until I got under it with the motor running.

it could the crank pulley itself. Take it off
and start it up see if the noise goes away