Fluke 893A AC DC Differential Voltmeter - high pitched whine

[mapleLC]

I have a Fluke 893A AC DC Differential Voltmeter that I want to use more often, but its making a terrible high pitched sound between 8 and 10k hz coming from what seems like the card identified in this photo.  Service manual link also.

When I unplug the lead to pin 2 on this board the sound stops.  What could be responsible for a sound like that?  I have never encountered something like this, so I was hoping it was something obvious.  I am not certain its this board, only the behavior of pin 2.


https://www.download-service-manuals.com/en/manual.php?file=Fluke-10130.pdf

[Kleinstein]

There are transforms for the reference signal and the recorder output, that run at 3 kHZ and thus in a well hearable range.  Some tone may be normal an hard to avoid.

[mapleLC]

It's quite loud, almost to the level of an error tone you might here.  It would drive a user crazy if you had to sit near it all day.

[Kleinstein]

I would consider the transformers the main suspects for a beeping noise. There are a few things that can make tranformer noise worse:
 - added DC background magnetization / asymmetric drive
 - increased drive voltage, so the core gets closer to saturation
 - the winding getting mechanically loose so it can move / resonate
 - rubber used for acoustic isolation getting hard.

Magnetistriction can produce twice the frequency of the voltage - so this would be some 6 kHz.

At least things like loose coils at the transformer can be fixed and some glue can also help to dampen mechnical resonances.