ok i was driven home and my motor started to make a noise so i let it sit. i got to lookin at and i think it is comin from the head. it was a real loud tappin like noise. it almost has to be a valve seat or guide has came loose. i thought at first it dropped a valve but it doesnt miss and it still runs fine. does anybody know about how much a valve job would be. or if it could be anything else.