... ... when I have the overdrive on and then turn on the compressor it makes a high pitch squeal ... ...
My first thought would be some sort of interaction with another component(s) of your rig. Is your bass active? If so, make sure it's got a fresh battery. Do you have Overdrive, Vintage, Bright or Contour buttons engaged? If so, turn them off. Turn off the horn (tweeter) on your amp, if it has one.
Re-start at very low Master level, no Gain, Bass, Mids and treble at noon on the amp and all controls on the pedal turned very low and bypassed. Get a low volume on the amp alone, then try each component of the pedal one-at-a-time, methodically. If each one alone seems to work OK, then start trying combinations, increasing complexity until isolating any problem.
If no problem is found with the pedal itself, then try adding one effect at a time on the amp until any problem is found. Likewise, if each individual amp effect passes, then start trying combinations. It's just a long, sequential, methodical process.
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Don't turn on the compressor.
Maybe @G-Dog might stop by.
Sorry, I'm not really a pedal person, as such. I prefer built-in effects, like on my Rumble Studio 40!
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How are you powering the pedal? I only ask because I had an Ampeg SCR-DI that would let out a squeal when on 9v battery and it got low.
That would be a logical troubleshooting idea, except the manual for the Fender Downtown Express gives no indication of the possibility of battery power. So, be sure the AC cord is securely attached to the transformer brick, and that the DC cord is securely attached to the pedal. Or try plugging into another electrical circuit, perhaps in another room.
Good luck, Sammy!
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