Sid is right. I’ve had buzzing that sounded like it was coming from the harp but turned out to be the bottom drawer of my metal file cabinet. Another time it was a loose screw in a hinge of a cabinet door (!) and once it was the metal grate cover on the heating/air conditioning duct in the floor. You just never know.
It really helps if you can get another person to listen with you and move around the room while you pluck the string so he or she can try to find the source.
Do get a copy of Carl Swanson’s book “A Guide for Harpists,” which is available from Vanderbilt Music and harp retailers. Not expensive and totally worth it, plus it’s a fun read if you’re into harps.