vicdad

I have a Spider II 75 Stereo head that has developed a problem. It has an intermittent noise generated in it. It is an increasing bass sound that builds maybe two seconds then dies away. Best way I can describe the noise is as a whump. It starts very low in intensity then builds for maybe two seconds then cuts off sharply with a slight pop . It's almost as though it is picking up some type of interference. Anyone ever experience such a thing? Bad transformer? Other than that, the amp works fine. I do like to use the amp for certain sounds, so I would like to fix it. Any help would be appreciated.