Do you know where to get replacement strips?The yoke is almost always what buzzes, since it's a little chamber of changing magnetic energy. Often these correction strips and the material that they are attached with go brittle and crack over time. Most Blast monitors end up with horrible convergence because the original strips have just gone brittle and fallen out - I have found shreds of the original strips just lying on top of my chassis as a result.
Every yoke I've had that buzzed was easily fixed with some cleaning, removal of old strips, application of new strips, and reinstallation. One thing I've not tried is impregnating it with the stuff they use on transformer windings, which finds its way in there with capillary action very effectively.