Strange warbling when using my Moto X Force in a slide-up wallet case

[MotoFanUK]

I have a Moto X Force and since the latest OS patch/upgrade it has been acting up. The latest thing is, when the phone is in the new slide case (wallet where the phone clips into a slide that I have to push up to use the camera) it keeps making an annoying warbling sound - a bit like a normal ringing ring tone heard underwater! It only happens when the phone is in the case and I slide up from down or down from up. I have to turn the ringer volume off so I can't hear it. Any ideas?