I bought a ready built QCX+ a few months back and after a few days of faultless operation it developed a weird fault involving very strange noises - digital pulses audible and horrifically loud sidetone. I spent quite a bit of time inspecting the circuit board for unsoldered joints, shorts caused by tiny flakes of solder and wire left after cutting soldered leads and all looked perfect.
My fault also involved microphony and I eventually tracked the issue down to the location around C24, an electrolytic capacitor. I found that by very gently tapping that perfect looking capacitor the fault would come and go. On inspecting the connections at the back of the board to C24, I could see that one lead had a fraction of a mm of wire poking out of the joint and the other was just flat solder - without a wire. I wobbled the part and one wire was simply sitting in the hole unsoldered, even though the builder had attempted to solder the lead and had simply covered the hole with solder. The lead was making intermittent contact in my case.
The OP's fault might be something like that. I assume that in my case the builder had snipped the surplus leads off, placed the component and one lead was a bit too short to get right through to the track and thus did not get hot enough to complete the soldering job when the iron was applied.
I had checked that board many times and every component looked like it was soldered properly. The microphonic effect might indicate something like that is the cause here.
It is worth checking anyway perhaps.