1994 Chevy Corsica Loud noise while in reverse

My car, while in idle, idles rather rough, but I have a feeling that might be the throttle control, seeing when I press the gas a bit it stops. My main concern, and question, is that when it is in reverse, it makes a loud clicking noise, well more of a thump thump thump, this noise dosent vary pitch or frequency with speed, and once you put the car into reverse it automatically makes this noise. I havent had this problem with any of the other drive settings. Also I noticed that two gaskets are corroded on the catalytic converter, and that there is an excessive oil leak, though the oil level in the car dosent change, what do you think might be causing this?