Two q's on my 05 Altima: what would make a tapping sound in the engine when accelerating ?

The other noise has happened three times: a loud clunk when going in (or out) of what I think is overdrive on a grade. Car is regularly serviced and radiator and AT was flushed and fluids changed this month. It has 80K, majority highway miles. I use the dealership service shop. The only thing they said was everything looked "fine." Is there anything else I should ask them to check or just wait and see if it clunks again?