Horrible noise from engine

bought a polo S sdi engine , horrible noisy engine , sounds like tractor with less pick up specially in 1st 2nd gear but in speed drive ok, when i press the cluth the sounds go down but in driving its keep making sounds is it possible flywheel? or gear box or any thing other
been to a mechanic suggested 600 pound to replace gear box n fly wheel where am not sure his dignostic is right ?
cheers all