If it's pulsating (about .5 sec on off sound), you're on windows 10 and use USB audio the cause is probably some windows update, graphics related. Get the latest graphics driver, set or choose basic settings with no gaming acceleration, and switch off power management for USB. IMO Windows 10 updates automatically screws up latter settings BTW, why i always switch it off and use an external program to select only the necessary security updates. -F