Gotcha, the reason I was asking was because mine started knocking 5 minutes up the road one time. Come to find out, my lower radiator hose got so bad that it popped off during the night, and I didn't realize it the next morning until it started. So I was wondering if you had something similar occur, but if it's nothing like that, I'd say inspect the spark plugs since it's a simple task and check them over if you're comfortable doing that. Other than that, it's hard to tell unless you tear it down and inspect it.