Check that the buzz really is coming from the harp
If it’s a metal string, check that the winding isn’t loose; this usually occurs near the levers
Check that the ball end at the bottom of the string is seated neatly against the string bar
If it’s a gut string, check that it isn’t worn with loose fibres
Check that the thick bits of gut at the base of the strings are at right angles to the string bar and aren’t touching each other; check all the way up the string bar, not just in the area of the buzzing string
If it’s still buzzing, check that the large nut inside the box at the base of the harp is very tight
If the harp buzzes only with the lever on, check that the cap screw fixing the lever bracket to the harp is tight and also check those around it
Hold the lever on firmly by hand: if this stops the buzz, then the handle may be worn and need replacing, or
The handle may need tightening on its bracket
The buzz may be caused by a lever handle on a different string vibrating if it is loose, so you may have to pluck the string and hold each handle in turn to check this
On some older harps, the stiffener strips going vertically down the back of the soundboard inside the box can vibrate loose – check they are secure
If none of this works, are there any other parts of the harp which could don’t seem secure – the screws in the base, for instance. If you can, tighten it!
If none of this works, which is highly unlikely, then we need to see the harp.