Just from a Google search
a lamination, winding or clamping screw in the transformer has come loose. Screws can be tightened, and you could maybe re-dip the transformer in magic lacquer.
the quiescent load on the transformer has increased, making it hum more (e.g. capacitor going bad, diode failing)
http://audiokarma.org/forums/index.php?threads/does-you-mc250-hum-vibrate.149641/
https://forums.stevehoffman.tv/threads/mcintosh-transformer-hum.74428/