I first thought it was the drum rollers (4392067 - Dryer repair kit) and/or shafts (2X - WPW10359272, 2X - WPW10359271), so I replaced them. The noise persisted, so I moved on to the blower fan. Once the blower fan was replaced the noise was gone. The old fan looks "ok", but I suspect that it may be warped enough to set up a vibration in the unit.