Could be a number of things, firstly you know it's not the amp.
OK, it could be crappy leads, that's the first and most obvious thing to try.
If it's not that it could be a grounding problem in the guitar or something in the effects pedals.
Try running the guitar strait into the amp, with a lead you know is good, and see if it's still there, if it is, try touching the input socket of the guitar while it's plugged in, if the sound disapears it's a grounding problem in the guitar, that's easy to fix, open the guitar up and see if there's any lose connections.
If it's not the guitar it could be one of your effects pedals, just try them, one at a time, no others in the loop, not even bypassed, untill you find which one it is. Once you've found out which one it is, save for changing the batteries, I don't really know what to suggest, but if you get to this stage, let us know, someone will be able to help.
If you have tried everything, and it still persists, try your equipment through someone elses amp, and try someone elses guitar through your amp.
Just one thing though, if it's only happening on the dist channel, are you sure it's not just because you have the gain up high? I know this sounds patronising, but you'd be suprised how many people have found that is the solution in the past.
