Exhaust? - I used to have one of those Nismo back boxes, and I think they must have made it from cheese.
It's unlikely to be a vacuum leak, but it's quite possibly fuel (from 20-year old hoses perishing) or exhaust (as above). If you only notice it after some spirited driving, there's a chance it's your brakes cooking... or the clutch slipping.
As Ben says, you need to be a little more specific.