how to make my engine on my passat quieter

im not sure i know where your going with this but if your hearing a loud rattling noise from the muffler then your muffler is shot, the muffler has a bunch of plates in it that directs the flow into a more quiet stream. some times the plates dislodge and causes louder muffler sounds and rattling which is the loose plates. If your sound is half way up your car good chances are the flex pipes ahead of the catalytic are broken. It is very common for the 1996-2003 modal cars from vw and audi to break after so much use.

Maybe your original engine had poor oil pressure, due to improper oil usage, which makes the lifters and cam tensioner noisy. And then the motor was replaced with another one that has poor oil pressure too? The oil pressure should be checked and if it is low, the oil pan needs to be removed for the oil pickup tube to be cleaned out. And the car needs the proper VW specification oil to be used so that this doesn't recur.