Cheap off brand psu's are often very loud. There are kits that include rubber gaskets and washers to isolate the psu and reduce noise from vibration, which might help some. You also could try replacing the fan in the psu, http://www.tomshardware.com/howto/20030609/power_supplies-03.html#silent_service_and_operation
tells how, but be very careful, there are capacitors in the psu that store powerful charge even when unplugged.
Finally, you could get a new psu. Zalman and other makers make psu's that are very quiet and/or silent (through passive cooling). The rating of the psu should be on the sticker, you can look to ensure you know its capacity and such.