I searched all over for an engine diagram to identify this part but cant find one anywhere. Anyway heres the problem.
When at idle in anything but nuetral, this part of the block makes a clanking rattling sound. Anybody have any insight? I'm mainly trying to get the name of the part so I can do more research into it. BTW im obviously a engine newb.

 

the part your circled is called a serpentine belt tensioner assembly. they can make a rattle noise after high milage. id check the serpentine belt for cracks and wear as well if your going to change the whole assembly , dont forget the pulley needs to spin freely without any abnormal noise as well.