dboyn

Hi I am new to the forum. I have a 2004 f150 4.7 that I purchased from an auction. It looked great when I bought it however I have already had the trans rebuilt on it. The odometer says it has 77k miles on it. I recently changed the oil using ford motor craft synthetic oil. I begain to hear a tapping sound the next day and changed the oil to Mobil 1. The tapping sound continued. It sounds like a valve tapping but my mechanic said it sounds like the timing chain guide. However he did not sound very sure and stated it would cost 1200 bucks to rip everything apart to find out for sure. Anyone have any other educted guesses or suggestions? Thanks.