Up until last week, when I finally found my way to the SPAH convention, I would have said, "buy a new harmonica".
It is entirely possible that is still the best advice, but I am now aware of a plethora of options. The most interesting one was to press out the revit on the rattling reed, ream & tap the hole on the plate, ream the hole on the reed, and put it all back together with a 1.5mm x 4mm bolt and accompanying lock nut. Oh, you may need to canibilize a reed from an old harp and then retune everything. Toolkit with rivet tool, and a couple hundred screws $299.http://www.hohnerusa.com/index.php?1625
Another option may be as simple as pulling the cover off and plucking the reed free from the plate.
If rattling is an accurate description you may have a cracked reed. Without the toolkit, or the hobby machinst's equivalent, buying a new harp may be the most expedient and economical route to take. But, hang on to that old Golden Melody for tuning & repair practice and future canibilization.
I have a couple of harps with sticking reeds and one that is rattling so perhaps I will update this answer in the near future. I am about to start attemptig to use everything I learned last week.