Powered monitor started buzzing noise.

Fazer

Hello, one of my M-Audio BX8a powered monitors has just started a permanent insistent buzzing of the tweeter.
They've been sitting on shelves working fine for 10-15yrs.
It's not the cable - it's the same buzz with nothing plugged in & with it's usual cable plugged in, and I changed the mains socket it's plugged into and it just the same permanent buzz.

Any ideas of something obvious that I could look to fix?

ToneControl

what frequency?
are you sure it's the tweeter?

I get some buzzing when a humidifier switches on, some household appliances can inject noise into the mains

Fazer

I can feel the tweeter moving.
According to app i just downloaded it's 1400Hz.
(I think the cross-over freq for the tweeter is listed as 2.2kHz)

Nothing has changed about my setup, it just started happening yesterday and it doesn't change.