One of the simplest things to try is a spare cord between the FX send and return jacks (or preamp out & pwr amp in).
These jacks have internal switches that route the signal through. They often cause problems like this and are a great place to begin trouble shooting.
If that doesn't help, try plugging your guitar straight in to the FX return or pwr amp in jack. If the problem stops, it is likely you have a preamp problem. If the problem continues, you have isolated it to the power amp or speaker.
Alternatively, you can run the FX send/preamp out to another amp and see if the problem is still there.
These jacks have internal switches that route the signal through. They often cause problems like this and are a great place to begin trouble shooting.
If that doesn't help, try plugging your guitar straight in to the FX return or pwr amp in jack. If the problem stops, it is likely you have a preamp problem. If the problem continues, you have isolated it to the power amp or speaker.
Alternatively, you can run the FX send/preamp out to another amp and see if the problem is still there.