.
This describes the signal transfer from the heads:
https://en.wikipedia.org/wiki/Rotary_transformer
Could still be dirty heads, or try demagnetizing the heads with a small
pencil sized tape head demagnetizer. I had one from Radio Shack,
looked like a pencil. We use to clean the heads on the drum with
Alcohol first, then a VRC Q-Tip full of Freeze (in those days Freon)
Worked well..... I think the cold temps caused the contaminants
to flake off the heads - Company Reps said to do it that way
so that's what we did....
https://www.ebay.com/itm/178145532551
VCR Q-Tips seemed to be made of the softer part of Velcro
not the hook part, and they were flat, and after Freezing
it, you placed it on the drum before the head and moved
it past the head with the direction of rotation only, not
up and down but moving as if it was the video tape
on the heads in normal operation.
You'll want to watch the system in operation as well,
be sure the tape is not being bent as it moves
past all of the tape guides. By Bent I mean
if a guide is moved, it's limits may push
on the top or bottom of the tape and you
can see the tape distort from it's flat
orientation as it moves... Not likely.
Don't turn anything.....
Anyway, the noise is hitting the blue key
on the piano, so it's got a specific
frequency note to it.... Check
the plug for the pickup where
it goes to the main board.
Dirty noisy contacts.
.