gpz750 engine noise?

Alright so i got my 82 gpz750 running but now i have a little noise that i can't quite figure out what it is. When i'm riding it out and about the engine makes a noise that sounds a little like someone hitting a hollow piece of pvc. Its really hard to describe but if i was to type it is a low pitched "foomp" sound, not like a backfire though. I know that sounds really goofy here but i would really like to know if anyone else has had a problem like this. i don't know if a cylinder is miss firing or what. please help! Thanks.