@RogerP
You can try using another HDMI cable, but there was an issue with some Ultra 4800 devices when it was initially released with this buzzing sound upon startup. I believe most users just replaced the unit to resolve the issue. (short of that, a System Restart, or rebooting the device, will also temporarily resolve issue until it occurs again.)
Roku Community Streaming Expert
Just another Roku user... I am not a Roku employee.
Insignia RokuTV, Ultra 4660, Premiere+ 3921, Express 4k+ 3941, Streambar 9102