Amp making noise on shut off

Sounds like a hissing noise from left channel? Causes? Meaning?
Meaning: nothing to worry about. Some tubes will do this and its not an indication of failure or sound quality.
I've seen solid state amps do this too. This is a bit more concerning than it is in a tube amplifier; it may mean that an active device is failing. If this is the case I think you'll find that it gets noisier to the point that you hear it when the amp is on- at which point it should be serviced, as the noise could be the result of an oscillation. I wouldn't worry about it other than that.
@atmasphere  sorry about misspelling. I have grest respect for your OTL Amplifiers :)
It seems the noise after amp shutdown could be a capacitor or two on the way to failure - I had a similar symptom in a preamp that was solved by replacing the caps...
Is replacing caps a factory repair job?
Any competent technician could do the job. Is the amp in warranty? If yes, I'd send it back for repair.