You sorta hafta look at the horizontal with a scope. There's an internal horizontal oscillator. It cooks along well below the real horizontal signal frequency wise. The Hsync signal from the computer comes along and triggers a new horizontal refresh line before the internal oscillator would on its own.
Lose the horizontal sync, and the monitor's internal oscillator runs for a bit but at a lower frequency (that you hear), the big brother circuit kicks in and says go to standby we ain't got the right drive signals.
Doesn't have to be a wire - but that's a place to start.
Any circuit board connection to the Horz or Vert sync signal amplifiers could cause similar symptoms.
Many monitors use single integrated circuits for each - the horz and vertical drivers. Both are usually high power devices. They are mounted to heatsinks. A common problem is the heatsink expands with the warm-up and stresses the leads to the device - eventually breaking the solder joint. Both heatsink and IC are mounted to the board - but expand at different rates with temperature. Re solder all the connections to those parts will sometimes fix that problem.
It is unlikely that anyone reading about it will give you the right answer - you are in the driver's seat and have to think it through.
I've fixed a lot of monitors and both the vertical and horizontal driver circuits fail frequently. (the monitor I'm using now has had both problems). Power circuits are another weak link - increase the
60-50 HZ ripple on a monitor PS and it starts modulating the 60-70 HZ free running vertical oscillator.
Nine times out of nine it is the construction technique and design that cause problems - very seldom a part failure (except electrolytic's in the PS). They "wave solder" the boards in mass production - but not all connections are happy with just a taste of solder. The larger leads and problem areas need more heat for a good solder joint.
Then we have the old ViewSonic monitors . . . they hang ten pounds of tweaking hardware (coils and pots for pincushion gun drive etc.) on the connector to the CRT - they all die prematurely
Very few component failures for me - but lots of bad solder joints.
If you could describe the sound better - that might be a clue.
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