The circuit is simple enough that there can't be too many troubles.

On to the FMEA...
Hum can be from two sources, the plate supply or the input. THe cathode counted as an input.

If it is the plate supply, I suggest MOSFET failure. It is not likely that a working CCS lets enough hum through. Check plate voltage, and if possible measure out-of-circuit gate-source resistance. it should be infinite, excepting some capaciatance( but we're talking DC...)

If it is the input, there are more troublesome things to check. grounding is too wide a net, look at individual ground points, and check physical integrity. Of lesser likelyhood, a filament-cathode short would cause hum of some, given that we're not running pure DC.
regards,
Douglas