Hum from power transformer?

[Jamus]

After turning my kaiju on just now, before starting music, I noticed a high pitch hum coming from the amp. When I put pressure on the bell end it completely stops the hum. It's loud enough that I now notice it during quiet passages in the music. Any ideas what could be wrong?

[Rocketman248]

Power transformers will vibrate.  Make sure your screws are tightened fully. 

[Jamus]

Thanks for the suggestion. After warming up/playing for about an hour the hum was much less noticeable. I did tighten the screws once it cooled back down and while there is still a very faint hum it's nowhere near as loud as it was at first.