loud kocking sound !!

[VINTY]

I have a 1999 450 chim which has all of a sudden developed a very loud knocking sound. It seems to be coming from the top of the engine towards the bulkhead and sounds like somthing has come loose and is banging against the rocker cover,oil pressure seems okay but i have not attempted to drive it so dont know if there is any loss of power. Any one know what it could be,valve maybe? and any ideas on how expensive it is likley to be?