When playing my line 6 Spider Valve Mk2 112 the input signal suddenly dropped out and all that was happening was a high-pitchedsqueal sound (not feedback).  I've unplugged all input cables and the sound remains, the master volume still operates to control the level of whatever the noise is, and it stops when on standby.  I've had the amp switched off for a good 15 minutes and tried again but the noise persists.  Any ideas what this may be?