Hope someone can help.  When I power this beast up it works fine for a few minutes and then I start to hear a "roar" like the reverb spring is moving.  I can cut some of it out by adjusting the bass.  Turning the reverb all the way down has no effect so I'm not sure it's a reverb problem.  Nothing gets hot and amp plays fine but it has that annoying roaring sound.  Any suggestions?
Thanks.