My altima 2.5s 2002 make noise of flapping from passenger side one mechanic say it is timing belt other say exhaust manifold please tell me how to find out

i went to mechanic today he told it is cat heat shield so may be by fixing it this thing will sound better

Yes, same thing happened to me, it is the cat converter hear shroud, usually the padding around the bolts deteriorates away, so you can try to pad it with something laying around, or pay someone to do it, but it's really easy to diagnose yourself, i promise. If you have a socket wrench your golden.