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.

Good luck
Matthias