I noticed a new noise coming from my 70's SVT. Have you ever had a guitar amp with a crappy spring reverb and tapped it with your hand? You know that echo-y internal noise you hear? That's what I'm hearing in my amp. I'm thinking of moving the head off the cab, as vibrations seem to cause this noise, as well as tapping the head or front plate itself. Playing loud, especially low notes, will set off this noise, which will last for a second or almost a minute. When the noise stops, it stops short, like it's cut off by some kind of gate. This is annoying, as I'm going to need it for my new band's first gig, which is on May 10th. I don't have time for it to sit on the tech's shelf for a month. Any ideas?