hi i need help i was driving down the road bare in mind it has had a new top end a year ago and it startd nocking and then gettin louder as i drove on and this was coming from the the piston area and really dont no what it is i have took top off and all the is is some valves and a weel and chain but can not get to the psiton bit as it may mess it all up so please could some 1 tell me what it could be and how much .!!! thanks luke