Both my M's make that sound, the 8 x 8 and the 12 x 12, especially the 8x8. I attribute it to the hours, the 8x8 has 4000 hours on it., but only on the low side of the box. 12 x 12, not so much. Always been there. I believe it's internal slop in the gear sets. Never anything in the filters or on the magnet in the filter housing/ The 12 x 12 has a tad over 2000 hours on it..