It's not unusual for transformers to hum. This can occur when large harmonics are present on the AC line. I had this problem with a CODA amplifier and called my local power company out to check the lines and they found that the 10KVA transformer on the pole was going in and out of spec causing higher than usual harmonics and voltage swings. Fluctuating voltage can cause this as well. Most transformers like to work at 110 VAC. When your line fluctuates more than 15 to 20 percent you may notice a hum in your amplifier.
Filter caps can cause hum as well, most output filter caps internal to amplifiers are good for at least 10 years, that's not to say they don't go bad sooner than that. Its just a lot easier to have your local power company verfy the specification of the power than it is to pack up and send a 150 pound amp back to the manufacturer.
If it is your power, you might wind up sending it back and they won't find a problem with it, because their power is fine. Good Luck, Indiana_Jones
Filter caps can cause hum as well, most output filter caps internal to amplifiers are good for at least 10 years, that's not to say they don't go bad sooner than that. Its just a lot easier to have your local power company verfy the specification of the power than it is to pack up and send a 150 pound amp back to the manufacturer.
If it is your power, you might wind up sending it back and they won't find a problem with it, because their power is fine. Good Luck, Indiana_Jones