This reproed again yesterday.
It was reasonably hot in the room, probably 80.
After the noise started, adjusting the volume did not change it.
Switching input sources killed not only the noise, but all sound.
Switching input source again did not restore sound.
After the 2nd switch, the TV was unreponsive to console or remote, and had to be unplugged.
At this point I guess I need to go to HP Support. Last time I searched this out, the link was dead, but I'll try again.