I had the intake manifold gasket replaced on my 97 4x4 last week,the mechanic said after he got it all put back together he noticed the engine had a loud tapping or clicking sound.He pulled the intake back off to look for something foriegn that may have fallen into it.He still has the truck and says he can't find anything so far.What could this be?It was running great before,it just had a coolant leak.