Rule out typical sources of noise that are not amplifier-related. Disconnect the guitar and cable from the amplifier inputs. If the noise stops, the problem is with the guitar. Plug the Dual Rectifier into a different AC outlet to rule out ground issues with electrical supply. Also, move the amp to a different location. If the level or type of noise changes, it may indicate electromagnetic interference is the culprit.
Investigate preamp tube noise. Use a tube layout chart to identify where the smaller 12AX7 tubes are located within your amp. Tap each tube with a fingernail. While it's normal to hear a metallic ringing noise from preamp tubes when you tap them, listen for a change to the problem noise, crackling or feedback. If one tube responds this way when tapped, replace it with a new tube.
Check power amp tubes using the same method you used to check the preamp. These are the larger 6L6 or EL34 tubes. When working properly, power tubes will not make noise when tapped. Replace any tubes that do make noise.
Check each channel separately to see if the noise occurs in one channel or one mode of operation. Logically tracing the problem using the tube layout chart may lead you directly to a problem tube. If not, swap out each 12AX7 tube with a new one while the amp is in standby mode. If the noise continues, replace the original tube and move to the next until the problem hiss is gone.