It is quite possible for a dirty chain to be quieter than a clean one. All that grease and grime keeps parts from clattering against each other and from transmitting sound as well. There a many reasons your drive train is noisy, and it's very hard to diagnose them online. You need a good mechanic to check out the bike and look for the following (not necessarily and exhaustive list)

-Rear derailleur out of alignment, including bent pulley assembly or derailleur hanger. I strongly suggest you have at least that corrected at a bike shop that has the proper tool.
-Jockey wheel play, rusty/dry bushings, damaged teeth
-Too little or ultra light lubrication
-Snaky" chain (weaves back and forth as it leaves lower derailleur pulley toward chainwheel when backpedaling)
-Some chains are noisier than others.
-Chainwheel wear (unless by "whole drivetrain new" you are including chainwheels)

Two others would not seem to apply to your situation
Worn chain or cassette
Slight misadjustment of rear derailleur

You can receive guesses here til the cows come home but the only way to tell for sure is to eliminate possibilities one by one with proper adjustment or substitution.