Strange Intermittent Buzzing "Wahh" noise appearing

Hi folks, I've got a strange thing going on - my latest Dulcimer is making a strange noise every now and again...

It seems to be occurring when I play a chord at frets 305 (DAd) and 304.

It's not always there when I play those chords and can start when I'm fingerpicking and strumming.

The noise isn't really a buzz but a kind of weird robotic "WAAHH" that just starts up; usually in the middle of a song.

I've tried raising the action at the nut and bridge so see if it was the frets and it still happens.

Does anyone have any suggestions of what it could be?