The notes out of my little Koch Studiotone sometimes are starting to have a slightly "strangled" trembling sound. It's so subtle, I sometimes think maybe it's just my hearing. But there is this subtle warbling sound that I don't think should be there. Anyone know what this might be? It's as if there is a far-away woman with a subtle vibrato who is being strangled gently and this sound is getting mixed into my playing. It's most noticeable when I play slowly on single note runs. Any ideas? Thanks.