There a couple of things I find odd with my car, and I'm wondering if something may be wrong or going wrong. when I let off the gas, the engine slows down really fast between 1k-2k rpm. Also when I stop at a light, I can hear a small knock from the back, it will do it once every 15 seconds or so, and seems to go away when I get moving or shut the key off. Im thinking maybe the return on the fuel pressure regulator (If it has a return style one) The last thing is when I shut the engine off, theres a kind of fluttering knock. I tried to record it but it wouldnt play back loud enough.

 

Also sounds like it could be a rattling exhaust.