Popping noise and vibration from the engine

Actually the 4 OR 6 cylinder engines for your Altima are not equipped with a timing 'belt' .. They have timing 'chains' which 'usually' doesn't require replacement unless worn and noisy .. There's a couple of possibilities for this issue but need to know > .. Did it have this problem before the repair work? .. Is it the V/6 or the 4 cylinder engine?