Lacking details, what car, mileage,, etc. It could be some carbon buildup in the engine causing pre-igntion. That was the case for 3 cars I have owned. It could be other things, but that would be my first guess.

http://www.agcoauto.com/content/news/p2_...

It could get worse and further engine damage could result to the point of the car being non-functional.

It wants to tell you a knock knock joke,

It wants out? How about some frikkin' details.

answer the door.