It’s hard to describe this sound, but it’s like a faint “dial tone” background noise. It shows up as soon as you turn the amp on and the tubes begin to warm up. It only seems to be audible when I’m plugged in to Input 1. When I plugged in to Input 2, it went away. The tone gets louder if you turn the Volume up on Input 1. Any ideas to isolate/troubleshoot further? I should note this started happening a week or two ago. It wasn’t occurring before this point.