Reed rattle on Hole # 1 draw is a very common problem. There could be a couple of reasons why this could be happening. The most common reason is the reed hitting the cover plate. The Hole # 1 draw reed is the longest draw reed, so it can and does hit the cover plate at times. This doesn't necessarily mean it's a defective harp; try drawing a little softer. Beginners sometime have a tendency to blow or suck too hard. If this doesn't solve the problem, you can check if the rattling sound is coming from the reed hitting the cover plate or not, by playing the harmonica without its covers. Removing the cover plates is easy, you just have to remove the two screws holding them. Hohner Special 20's have the same size cover plates on all harmonicas, regardless of key. Lee Oskar alleviates this reed rattle problem by using deeper cover plates on low keyed harps such as A and G.
If the reed rattles even without the coverplates, then the rattling sound is most probably coming from a burr (or some type of debris) in the reed slot. I have had a Hohner Special 20 in C which had the same issue but it is very rare.