Sencore CR70 whining sound - cause?

[cj_reha]

Hello all -

I recently purchased a Sencore CR70 CRT tester via eBay and it arrived today. Whilst in the middle of testing every tube in the vicinity, I've noticed something quite annoying - it emits a loud, high-pitched whine which changes frequency with the adjustment of the filament voltage. The tester itself seems fine, and all CRTs I've tested with it have accurate emissions and shorts readings similar to ones I was given on my old CR168. I also tested the rejuvenation function on a weak 12LP4, and it successfully brought emission readings into the green range. Is there a known cause of this? Am I dealing with something simple like a dying capacitor, etc?

Any advice would be greatly appreciated.

-CJ