Fender mustang mute sitar twang

Not fret buzz (sorry, obvious), not a loose tuning key (had that happen more than once), not minor contact of the pad? I have had a pad on G barely make contact with the string. On close inspection it looked like it wasn't touching it but when I played it would very slightly buzz, when I forced the mute away the buzz stopped. I had to trim that one mute to get the action I liked.

 

OK, was gonna ask if you were crazy. Are you hearing it amplified or just unplugged? One last place you can check is the nut.

 

I had a bass where, if I fretted too hard, the "back part" of the string (between fret and nut) would ring out, as well. This is not picked up by the pickup, but I hear it anyway.

Also, "false harmonics." If you pluck at the harmonic point of a fretted note, you often get that harmonic coming through on the note. Octaves and 2nd octaves (12th & 5th fret on open strings) are good for that.

Next, muted strings ringing a harmonic. If a finger, or a mute, is touching the string at the proper point, harmonics (or drone tones, as well!) can come from sympathetic vibrations.