• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Noisy transformer

let me start by saying I am very new at this DIY audio stuff. Maybe this is something I should know, but I don't.

some time ago I built an aikido pre amp. I got the all in one board and parts from glassware audio. I used an Antek toroidal transformer 300 VA 230 volt. So far so good.

recently I finished a salas phono pre amp and shunt regulator. Still good. I noticed the phono pre got quieter when I put it's transformer in it's own box

The Aikido is fairly quiet, but not dead quiet, so I figure I should put it's transformer in it's own enclosure. I go ahead and find a steel box that just fits over the transformer and still fits in the preamps chassis. Great I think until I turn the thing on. The transformer buzzes and the center bolt gets hot?! I unplug it and check that I didn't short something out. I didn't. Take the box out It works fine put it in and it buzzes. Tell me why.....
Thanks, Evan