Blackstar HT Studio 20 on the bench today@shroproc…

Blackstar HT Studio 20 on the bench today@shroprockdoctor.
Initially couldn’t find the reported issue, a buzzing noise that appears once the amp warms up. Played a guitar through it fot 20 minutes still nothing. Left it running with no jack in the input & went for lunch. Returned 40 minutes later to this.