Do you do any electronic repairwork? Do you have any skills in electronic troubleshooting?
The first thing I'd do, is open the chassis and blow-out any dust or animal hair with compressed air. If you have pets, a lot of times the pet hairs get into electronics and covers the components, causing them to overheat.
This sounds like a heat-related problem to me. Doing the above, might solve the problem. If not:
After blowing-out the dust and whatever from the chassis, I'd take a can of 'freeze-spray' and individually hit each semiconductor. When you find the one that is overheating, cooling it down, will cause the sound to immediately
come back. Identifying the overheated component.
