Richard- Since you didn't detail that the noise was specific to one channel, I'll assume that it is occuring in both channels. With that said, swapping tubes from one channel to the other may not provide any further insight into the problem. However, swapping or just reseating the tubes will tend to "clean" the tube pins and socket contacts which is highly advisable. Not knowing the particular amp, I can't offer much more than the tubes may need replacement. I would start with the small signal tubes first, before replacing the output tubes simply because of the cost associated with them. FYI- New Old Stock (NOS) tubes are usually preferred sonically, but can greatly increase the cost of retubing. Output tubes are best acquired in matched sets to help insure longer life, maximum power output, and hopefully equal aging characteristics. You might also consider having a few spare small signal tubes on hand that are known good, to help in the troubleshooting efforts. Goodluck.