I have an inexpensive (entry level) Weltmeister 607 Anglo Concertina DG 20 on which I've been learning. While it is not a high quality instrument, it has been working adequately for me and the notes all had a pure tone. Suddenly, today, I noticed that it had developed what I can only describe as a tremolo. The notes seem two waver across a small harmonic range, or perhaps there are overtones on both the D and G rows. It is most pronounced when I'm expanding the bellows but I think I also hear it when compressing the bellows (although I'm less sure about that). I took off the plates on both sides and examined both reed boards and I also blew them out in case dust had lodged in there. I didn't see any obvious problems. When I put it back together, it seemed to play normally for a little while and then the problem came back. Any ideas would be much appreciated.