[Radio Wrangler]

With that transformer mounting method, use insulating 'top hat' bushes on the screws. Otherwise the screws and the outer laminations (and the metal chassis) create a conductive loop through which part of the flux of the transformer passes. It's not obvious, but it gives a shorted turn effect. It increases losses and acoustic noise, but there is often bad enough contact and enough series resistance to stop it being dramatic.

David