I just bought a used Yorkville 400B head. When I tried it out, it was fine during the short period I tried it out. I was running it today for a few hours and noticed a high-pitched whine coming from it. I turned it off and after it cooled down, I tried it again and it stopped whining. Totally silent operation. I have the lower wattage version Yorkville 200B and it does not do this. Any clues as to which component may be faulty?