5150 power source interference noise

[musicforlife]

I got another 5150 with original 60w power source but this one makes weird high pitched noise constantly when the letters appear on screen (it doesn't make the sound with only underline cursor on screen). It's some kind of interference noise because it reacts to everything on the screen and makes noises accordingly. When I do dir command, the listing makes rattling/shaking noise and when the dir command is finished it makes funny "blip" sound etc. I also tested both CGA and MDA that it's the same with both of them and without having monitor on.

What could cause this and how could I fix this?

I took some video of it:

https://www.dropbox.com/s/ms6nuz7dnicsbb4/VID_20200531_000942.mp4?dl=0

 

[modem7]

The way you shot the video suggests that the 'unusual' sound is coming from the PSU (rather than say, the motherboard's speaker). Is that so?

Do you have another PSU that you can try?

 

[modem7]

Are you positive that the noise is related to screen content? You are doing a directory listing of a floppy. When that happens, the spindle motor on the floppy drive turns, raising the amount of power that the drive draws from the PSU. For a Tandon TM100-2 drive, that is about an extra 7 to 8 Watts (info source at [here]) being drawn from a 60W PSU.

At [here] is a thread where the cause turned out to be a deteriorated PSU; worked okay at low power consumption, but not at high (but within spec) power consumption.