What noise? The slap/thwack of the piston head hitting the cylinder head at the end of the shot? The whine of the gears? The noise of the motor? The pop at the muzzle? Got to be more specific.
There are a couple of different types of "silent" piston heads. They do change the sound of the shot. Some guys swear by the sorbo pads...I don't use them myself. This is where most of the sound is made.
A softer spring would obviously help.
Better gears/nicer shimming will help take the noise out of the gears.
There's not a lot to do with motors other than to try a different one.
Since most of the noise is from the mechbox...a suppressor's use is limited. It will change the pitch of the sound...some setups are more of a dull thump, others don't seem to change at all.
Accuracy (doing it backwards is just guessing...start with 1 and go down the list):
1. Ensure compression is good/consistent
2. Use heavier quality BB's for accuracy
3. A nice hopup rubber will help put an even amount of spin on the BB's
4. Tightbores will help
If too much hopup is applied you can change the stiffness of the rubber, the stiffness of the nub. When you assemble just the hopup and barrel...with the hopup off and looking into the chamber...you shouldn't see any protrusion into the barrel, or at most just a little bit of a bump. Swap hopup rubbers/nubs/barrels to get the right fit. If it's just wonky no matter what you do, you can mod the hopup arm to let the nub sit a bit higher (= less hop)....it'd only take a sec with a round file.