I have recently set up a Pi 4 (4GB) and a Hifiberry DAC2 HD. All is fine except for one strange issue - After playing quietly for a while, sometimes when I lightly touch the casing (e.g. to see if it's running hot) the volume increases to a loud level. It doesn't happen every time and it doesn't seem to be an earthing issue. I can reset the volume to the original level and the unit responds but why does touching the casing cause this effect? Any thoughts?