Noise in the drive channel,that is not there in the clean channel is usually from noisy tubes or dirty/loose sockets.It may actually be there in the clean setting,but isnt as audible because of the extra gain stage in the lead channel.All pre amp tubes have a certain amount of "microphonics" some more than others,when you add a gain stage in a lead channel these noises are amplified an additional time,if you will,by the extra stage.Try rocking the tubes in the sockets one at a time with the amp in the "dirty channel" and see if the noise becomes more apparent,if you get additional noise when rocking the tube,just move it in a circular fashion in the socket,cleaning and re-tensioning the sockets will help.Also tap each tube and see if any noises show up,this will indicate a noisy tube.A noisy tube in a high gain amp isnt necessarilly bad,it just isnt good in a high gain circuit,it may very well work fine in a Fender type amp that isnt "high gain",so dont throw it away just try to replace it with a less noisy tube.But remember some noise in a hard driven lead channel is fairly normal,it is very hard to completely eliminate it,especially if you have the drive "dimed".