This problem sounds interesting indeed. Oscillation come sto mind...
While I'm sorry I can't come up with a solution from here, but maybe the following steps can help you isolate the problem:
1. Effects pedals & cables:
Try to find the effect that's causing the mess. You could start with only your 1st pedal, then add No2, No.3 etc until the squealing starts. Also keep an eye on the cables and replace cable No.1 with No.2...etc...
Some older pedals (wah, fuzz) can play funky games when used together.
That's why Geoffrey Teese uses a buffer for his RMC wahs.
If you use batteries, an almost empty block can also cause squealing...
If you use a wall-wart /daisy chain, try batteries for a change...
2. Guitar-Check Pups:
(I don't know the Sensors too well so this might not apply...)
If not properly waxed, some pups tend to squeal at higher volume - think 70ies Telecasters. Plug directly in your amp & try at the volume level you normally play.
3. Amp-Check Valves & Circuit:
(valves only if it has any...)
Sometimes a circuit reacts bitchy when getting older. Try switching the amp to 'play' and tap any valves with a pencil (softly). If you can hear this sound through the speaker, replace the valve. A doom-bound valve can cause high-pitch squeal feedback.
It could also be that you accidentally overpowered the (pre)amp with an over-the-top-boosted signal from your distortion effetcs, or some parts (grid-resistors or the like) have worn out. In this case (amplifier self-oscillation), I would take it to a tech...unless you want to fry yourself with deadly voltage - the voltage remains inside the circuit even after unplugging the amp until you drain the caps, so please be careful!! Working on the valves is OK, as long as they're not hot. Keep in mind that your amp might need re-bias if you have to change the power-amp valves.
4. You could google 'amp high pitch squeal' to find out you're not the only one with this problem...
Seriously , I would think your problem lies probably with either 1. or 3.
Maybe I could help - good luck!