I need to lose that Hummmm

[Jebem]

I'm glad You found the problem.
That vibrating bells are a common problem with so many less than optimal transformer designs.
The magnetic flux must be strong, but the used E/I lamination core is not being able to restrain it inside the magnetic circuit. This is a known problem with E/I cores, but the best designs, despite more expensive, can handle it really well.

Your amplifier looks really nice and well done, why don't you ask Sowter or SJS in UK a price for a custom transformer?
My 211 Se amp is using a SJS custom made power supply transformer (500VA) and never got any problems after 10 years of good service, and believe me, it is running at about 90% of its capacity, so the design is that good.

Cheers.
Jose