Rheem Prestige series with variable speed blower has suddenly (?) begun making a low rumbling noise when it's running at any speed. The noise is not periodic but random, and reminds me of the sound of the gas flame as I hear it through the wall of the furnace room, but of course the a/c is on (here in Los Angeles).

It makes the sound if I spin the blower wheel by hand. Given a half-hearted spin, it makes the noise once before slowing down and stopping. Doesn't look particularly unbalanced, turns freely.

Any clues?