TH100 Effects Loop problem

[Strat1127]

Very frustrating head-scratcher. My (approximately) 10 year old TH100 seems to have a developed an issue with the effects loop. My rig has always been dead quiet but now has developed some high-frequency noise when idle and the amp will sometime feedback uncontrollably...especially when using reverb or delay. I have swapped out all the effects one by one, tried different cables, new high-end power supply, changed all my pre-amp tubes. I've also tried 3 different noise suppressors. When I remove all effects from the chain and just run the noise suppressor everything is fine...adding just one effect will create the problem. I also have a TH30 combo and it doesn't appear to have the same problem.

My signal path: Guitar, fuzz, tuner, wah into guitar-in on my ISP Decimator G-String then out to amp input. Effects loop from amp into G-String effects loop to cleanboost/buffer, Chorus, multi-modulation pedal, pitch shifter, reverb, back out to amp.

Any input is appreciated. Thanks!

[Hubaxe]

Change the effect loop valve/tube.
On mine I've changed all the 12AX7 + 12AT7 with quiet and carefully selected ones (from Tube Amp Doctor)
Quiet again.