Hum from the unit itself is indicative of one of four possibilities.

1. A damaged transformer would provide a constant hum that would not go away without repair. This is usually caused by delamination, that gets worse over time, due to one or more of the remaining three causes having been ignored for some time:

2. DC on your AC Mains. This is usually from equipment with some sort of motor - air conditioning units, hair dryers, power washers, fridge/freezer, etc. If you have DC on your AC mains, you would either have to find the offending device and stop using it, or purchase a DC filter.

3. The use of devices with cheaper TRIAC switching. This would not be solved by the use of a DC filter and you would be forced to find the offending device. The most common cause of this is lighting with cheap dimmer switches built in, or external dimmer switches on the wall. TRIACs are also commonly used to regulate the speed of ceiling fans and box fans, so those should be checked too. I'd start by trying to think of what has been added to your electrical environment, or if nothing has, has lighting or a fan been put on a different setting than previously? Once you've found the device, you'll find most of these cheaper circuits have different positions of the switch where one will specifically be problematic. If this is the case, you can avoid this position of the switch, or you'll need to avoid the using that device all together.

4. Voltage sag. This can be caused by too much load on a circuit; e.g. too many high current drawing devices such as amplifiers, subwoofers, air condition units being used. Relieving the load on the circuit will generally resolve the transformer hum. This can also be caused by brown out and brown out like conditions either in or outside of your control. This could be you turning on too many devices at a time that have a turn on surge, such as power amplifiers, or this could be a town induced brown out. If this is the case, we would recommend a power condition that offers AC line regulation; ideally, line regulation would kick in almost immediately after your line voltage drops. Buying a conditioner that says regulation or protection against voltage drops, but simply turns off when your voltage drops 20 or more volts is not a solution.

Please do not ignore this issue.

Toroidal transformers use an adhesive-style finish similar to a varnish to keep the natural vibration of the windings from making noise. If you ignore the issue, you can end up damaging/delaminating this and causing the amp to make noise all the time.

Number two and number three above are the most common cause of hum with an amp utilizing a Toroidal transformer.