Headphone Amp - LOUD buzz

I’m completely stumped and wondered if anyone has come across this before.
Last few days my new Boss IR-2 was absolutely silent through headphones. I was using exactly the same setup I am today: Telecaster into the IR-2, headphones plugged straight into the IR-2, same room and same mains socket.
Today it’s developed a really loud harsh electrical buzz. It’s much more than normal single-coil hum. It does it on my humbuckers too.
The strange thing is that I actually bought the IR-2 because my Blackstar ID:X had exactly the same problem through its headphone output. The IR-2 solved it immediately when I got it home but is now replicating the same problem exactly.
It goes away when volume on guitar is turned down so must be some sort of electrical interference? I don’t get why it is intermittent - it was literally fine and silent yesterday! Any suggestions gratefully appreciated as it is driving me mad! Don’t want to return the Boss as really like it and seems to be something about my house anyway.