No one can be 100% certain without testing, but with that said, by far the most obvious likelihood is transformer hum caused by DC in the AC line.
The fact that the hum is mechanical from the amp and not coming through the speakers is a dead giveaway, and the fact that the transformer is a large toroidal unit makes it even more likely since toroidals are well-known to be especially susceptible to DC-induced mechanical hum - and the bigger the toroidal, the more likely the hum will be audible/loud.
The troubleshooting steps are, as some have noted above, pretty clear.
1. Make sure the large screw in the center of the transformer is fully tight. IMHO it’s unlikely this will be the source of the problem, but it makes no sense not to check it first.
2. Try turning off or unplugging any fluorescent lights, lights on dimmer switches, TVs, fridges, or other large appliances in the house. Again unlikely to fix it imho (DC can come from a neighbor’s house or just the overall grid in your area), but worth a try.
3. Buy a DC blocker. They only do one thing, but they do it exceptionally well. If it’s DC in the AC line, you’ll confirm it instantly - DC blocker will reduce the hum 90-100%.
Good luck!