Hey guys and girls, hope all is well!
I just bought a 1990 240SX Fastback with 80K... Great car with only minor issues. Ill post some pics for you guys to flame my car

.
However sitting in my car the other day I turn on my beloved AC which works awesome. When I do that my car shakes and I hear a rattle, Both are intermiten. I look under the car and see a metal thing around the exhaust (resignator

) which is making the rattle.
After I turn it off i get idle surges that range from 500-1500 RPMs that feel like the car is about to shut off. After about 10 seconds they stop and the car is normal again.
Ok Nissan Gods

how do I fix this crap?
Thanks a ton for any response and I will post some pics after I get them developed.