part identification and loud rattling

need help id'ing this part:

http://i-fix-it.net/sub1993j.jpg

im pretty sure there use to be a belt around it. it snapped?

not sure.

before it snapped, from that same part, i would hear a loud rattling.

ESPECIALLY when i backed up. when running, it vibrates alot and rattles like crazy.

anyone? :frown:

Welcome to AF.
That is your belt tensioner.
Start with a new belt and belt tensioner.
Ans look other pulleys and stuff over.
MT