Why is my car making a rattling sound around the crankshaft area? on 1998 Nissan Sentra

When I rear up the engine the sound is not present but when it go to a normal idle the sound comes back.

2 answers
1- Have 'entire' drive belt systems checked, can remove belts, one at a time, to see if noise stops.
2- Possible cat. converter rattle.
agre with wetry or poss frt crank pulley falling apart