I have a TSL and found the same problem, along with fizzy buzzy distortion on the crunch and lead channels. I have found that these heads are very sensitive to the input power they are feed...they act pretty tame with good quality clean power, however with bad power or in an area with some electrical noise they behave like a 2 year old on crack. Fixing the problem will likely require some circuit modifications, so depending on the age of your head you might blow your warranty unless it's looked at by a service center. I'd take it in and let someone tinker with it. It could be as simple as adding some basic filtering circuitry but the trick is where to put it....these heads have seven printed circuit boards inside and there's not a lot of extra space laying around.
I sent my head to Voodoo Amps for some customized modifications and the noise is gone...he fixes all that as part of his basic service. It is tons quieter now than in stock form and it sounds incredible and it is extremely resonsive and dynamic.