Nissan Sentra, White smoke

Ok, I don't know a thing about cars... I know where to put the gas, and I can change my own oil, thats as far as my knowladge extends. Thats the reason I keep my wife around, she's a car chick, and can handle most problems that I get.
This one she dosn'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 wierdly, 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 probibly wasn't a big deal since it wasn't continious 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 rusteling.
I don't know if it's an exaust problem, or somethign more serious.
The car was EChecked in Febuary and passed with no problems... my roomate says that there is a small hole in my exaust though.

Any help you can offer will be greatly appreciated.
The car is a 1990 Nissan Sentra.

Thanks
MOT