Hey Sci Fi,
I bought an inexpensive piston compressor and the racket about drove me nuts. Definitely get something with an air tank, that way once it gets to pressure it turns off. You can even buy an inexpensive (~$70 contractors compressor at Menards or Home Depot) but you may need fittings to get your ab airhose to fit to the compressor outlet.
An inexpensive way to have complete silence is to buy an 'airtank', a pressure tested steel cylinder that is designed to be hooked to a compressor, filled and then disconnected. I've seen those for $ 30 or so.
Remember you need to have a decent regulator to control pressure, and a moisture trap.
I have actually gone to purchasing CO2 tanks and using my beer dispensing regulator to run my ab. Works great, perfectly precise on pressure and no noise. Down side is it just runs out of gas, and then you're out of luck until you get the tank filled.
Lots of options, but depending on your budget you can go in different ways.