Another source of the noise to keep in mind is the ringing of the tank itself from the pump vibration. Tap on the tank with your knuckle and listen to the ring. Now, dampen that vibration by grabbing hold of the tank and tap again. You'll have a thud instead of a high frequency ring. I've been wondering what effect wrapping the tank with Dynamat or something similar would have.
I've recently mounted my 60gal upright on some rubber tire swivel casters from Harbor Freight to make it mobile, so I can move it and the noise outside when the weather is suitable. Just being on casters lowered the sound level somewhat by reducing the vibration transferred to the concrete floor.
I've also considered moving and mounting the pump/motor combo outside or just making that portion mobile for a smaller package, say on a furniture dolly. If mounted permanently outside, the trick here in FL would be to protect it from the rain, keep the insects and frogs out of the motor, pullies and belts and isolate it's vibration from whatever I mount it on. (I guess better to mount it to a block and concrete structure than to stick built.)
Keep in mind the pressure switch needs to stay with the main tank, or a supplemental resevoir needs to be provided at the pump to avoid also running the electrical back to the tank along with the air. If you just "T" the pressure switch into the supply line, the points will start chattering or hammering on and off as you get close to shut-off pressure. DAMHIK
Mike Bynum