2003 Subaru Forester rear end noise

It's been a rough winter with record snows and now pot holes too large and numerous to avoid. I've started hearing a "rumbling" noise in the rear of my forester. My first thought was the spare in the cargo area was loose. Not the case. I have new tires with alignment in the past 2 weeks. The noise is frequent but random-not associated with speed or road condition.