....then the windings on a string are loose or something in cavity of the harp is loose. I spent days once chasing down a spring inside caught on brace. Just one of them things. BUT! There are also harps that buzz strings for no discernable reason. I have an Airline that does jusrt that.
I will concur with beeliner about 21 bar harps. Others won't. I just don't like them. I'd rather carry two that worked well with fewer chords than one I wasn't pleased with no matter the chordal convenience.
If the bars play what they say then I'd bet someone bought a harp with a lot of dropped felts and just reglued them with Ducco or Silicone II.
I'd look for an Older 15 Bar Appalachian were I looking...