Josh and others have mentioned Dynamat sound proofing matting helps a lot

i am using air solenoids on some,on others i make my own servo's that i found out noise is a lot less and dynamat is great also

DC motors and Hbridges

yes DAVE thats what i use to make my my own gear boxes and using a servo board

pneumatics Are nice and strong. They work great but were also noisy. They hiss and then you have the air compressor noise.

Not using a compressor,only a small tank,another item is using lego design type using a motor and syringe. So almost no noise and only reason is that on my johnny five project i need it to match johnny five solenoids on the head design,if you look at my project its on page 3

my johmmy five project its the ones in orange,ones i got is stainless steel and painting them orange to match them

Only need a small tank and can add a compressor like lego servo style to fill tank when get empty