Approximately 10 minutes after a power cycle(power on) on Rpi4, hdmi sound get constantly distorted in about 2-3 seconds intervals, I need to reboot to get it fix and then works again for another 10 minutes and so on. Ubuntu-Mate 20.10 is up to date.
Tested it with Rpi4 8Gb Rev1.4 w/USB3 boot(Kingston A400 256Gb) with Samsung LED TV 58 inches or Samsung QLED 75 inches running at 1920x1080(1080p)