1990 Nissan Sentra white smoke

1990 Nissan Sentra: Ok, I don't know a thing about cars. I know where to put the gas, and I can change my own oil, that’s as far as my knowledge extends. That’s the reason I keep my wife around, she's a car chick, and can handle most problems that I get. This one she doesn’t know what to do about though. I was driving on the highway and looked into my rear view mirror and noticed a single plume of white smoke come out the back end of my car, it didn't make any funny sounds or behave weirdly, just a single blast of smoke then it stopped and didn't happen again. I called my wife, but she had no idea what the problem was, I figured it probably wasn't a big deal since it wasn't continuous smoke and just kept on driving to work. As I pulled into my parking lot it happened again though, and this time I think I heard a funny sound that I could only describe as maybe a rustling. I don't know if it's an exhaust problem, or something more serious. The car was Checked in February and passed with no problems. My roommate says that there is a small hole in my exhaust though.