My Thermaltake Toughpower 750W makes a faint buzzing sound the moment I plug the power cord into the unit. This occurs when the power switch on the PSU is both on and off. I have tried disconnecting the PSU from the MB and it still makes this buzzing sound when plugged in. I tried using a different power cord and a different surge protector, but nothing changed. I guess I am concerned because I never noticed it before. My ear needs to be about 6 inches from my PSU to hear it, but I never recalled hearing it before. I first noticed the buzzing sound after having dusted my computer with compressed air. I blew some air into the PSU from about 3 inches away. I also ended up disconnecting my Radeon 3870 X2 graphics card, and I am 100% confident that I reconnected the 6-pin and 8-pin power supplies firmly and properly.
I've read around that PSUs can make buzzing sounds when they are heavily taxed, but would they still do that with both the PSU and the computer powered off?