I have a very fast knocking sound from the passenger side of my engine, but its only when I'm idling. Doesn't sound like metal on metal, just plastic on plastic. Any ideas?

 

does it come and go?

 

I do notice that sometimes its not as loud, but its always there, and only when I'm stopped.

 

Your exhaust could be hitting the cross member. If your transmission mount is sagging and old it lowers the exhaust with it. Put it in park and go around to the passenger side and look under at where the exhaust pipe passes over the cross member.