I'm having some problems with my rig right now. I've diagnosed the simple things I could think of like cables and the routing of everything and still having this problem.

My problem is when I go from not playing anything to playing something I get this loud annoying high pitched squeal from my amp. It only happens when you first strum and then goes away. It also happens if you happen to barley hit the string or tap the pickups.

My setup is as follows....

ESP EC-1000-Sennheiser Wireless- ISP Pro Rack G Decimator- SKB Pedal Board- Tuner- Tubescreamer- Boss Chorus- Memory Boy- Amp Input. Channel two of the Decimator runs threw the effects loop of my amp.

My amp is a 6505+ and ran threw an Emperor 6x12.

I've also noticed if I use my chorus pedal on the lead channel it makes a horrible sound as well. Not sure if my pre-amp tubes are going bad or the power tubes but it hadn't done this before with this setup.
Try using without the wireless (ie, guitar, cable, amp)
Just a suggestion, not too in the know with this stuff.
^this. Start from the top, and add in each part of your rig one by one.
Telecasters-->Dunlop Volume Pedal-T1M Pearl-AMT Japanese Girl Wah-Line 6 M9-Ibanez DE7-EHX Cathedral-->Mesa Boogie Nomad 4x10 combo
I had this problem at one point. It was so annoying! It was a faulty cable, from my pedalboard to my amp. Replace cables and see if it works.
Main Rig
Korg Pitchblack > Digitech Bad Monkey
Peavey 6505+ 112 w/ 2x12 (Celestion Vintage 30 and WGS Veteran 30)
[Effects Loop] Boss GE-7 > Boss CE-5 > TC Electronic Flashback