Problem with the S.E.X 2.1: Weird noise.

[hungdaubub123]

I also have another problem. Recently I noticed that my Bottlehead S.E.X 2.1 makes some annoying rings. That usually happens when I forget to turn off the DAC (the Bifrost DAC). After a while, when I turn on the S.E.X (the Bifrost DAC is still on, I forgot to turn the DAC off before) then it makes some noise and it rings (the noise is from the top plate, the tube, the transformers or any parts in the amp, but not heard in the headphone)
Has anybody faced this problem before or any ideas to fix this problem?
Thanks everyone.

[Doc B.]

Try tightening the power transformer mounting screws/nuts. While you are at it, make sure all of the chassis screws are tight.

[hungdaubub123]

Thanks Doc B.
However, I think that the noise doesn't seem to be from the loose screws/nuts. It's more like, an electric jam in the circuit or something like that. It happens when I didn't turn the DAC off before, then when I turn on the amp, the noise starts up.
Again, thanks "famous expert" Doc B.
Thanks all for helping.