is it a slow knock or a fast knock?
Slow knock is bottom end.... fast knock is top end.
If it's a slow knock then I would once again advise getting some new rings installed.

Seeing how it isn't coming out of the exhaust "that much" it's the start of your rings failing. replace them before it becomes excessive and embarrassing. Trust me i've been there and done that.

No better feeling then hauling ass to pass someone and covering them in a James Bond Smoke Screen. ^-- Sarcasm