I have a pair of restored model 20s that are fantastic but unfortunately one of them has developed a rattle when playing low frequencies at high volume. The speaker doesn't sound blown, but its almost as if something has fallen out of place and is vibrating against the cone. At lower volumes the sound disappears. Pulling off the grid there is no visible damage and the foam looks to be in good shape. Any ideas, advice, or suggestions would be very much appreciated. I don't really want to open this baby up until I know what I am planning to do.