very annoying, loud tapping/clicking from engine upon immediate start-up, then upon acceleration only. Read some from forum(s); checked the exhaust manifold, found a missing top/left bolt..replaced and tightened other manifold bolts along with visual for cracks...no luck, tapping still there. Tried the 'ole screwdriver at the end of a hose diagnosis trick..can't find the sound. Figured with that method on the valve cover, a sticky lifter would be easy to hear...nope. sound coming from back of motor, mid way down i think. Any ideas or similar discoveries out there? I'm frustrated!!