Antec signature 650 noise question

Hello, I've recently noticed that my Antec Signature 650 psu makes a faint noise while my computer is turned on. It's not a fan noise, as I jammed the fan for a few seconds and the noise was still present. The noise is a faint but constant buzzing, humming sound, not high pitched or whining, and can only be heard if I put my ear an inch or two away from any part of the psu, and it is only present if the computer is turned on. Is this normal operating noise for a power supply, or is it something to be concerned about? My system is stable, I have not been experiencing any strange errors, and I only noticed the noise because I had my head close to the case, so I don't know how long it's been occurring for.