alu foil will not work to insulate because the reason for
the 50HZ hum is magnetic induction so you have to position
the transformer in a position that creates the least
magnetic field intensity in the area of the PCB
Experiment: Try to hold the transformer in your hand and
turn it and move it around in relation to the pcb and listen
to the hum...
Since I got many mails from successful people that fixed it
by exactly following my guide I assume that my understanding
of the problem is probably right.
But they might have changed the design etc.