#29321, "RE: humming noise after alignment, kind of." In response to In response to 9
Maybe the alignment is just coincidental. I had a humming issue that sounds exacly like the one you describe. The humming would go away if I turned right even the slightest bit but continue no matter how far to the left I went. My problem ended up being a bad wheel bearing. Strange thing though...the humming sounded just like it was coming from the passenger side but it turned out to be the drivers side hub that was bad. Go figure
Not sure if thats your problem but something to check out if the tire swap doesn't pan out.
I had a humming issue that sounds exacly like the one you describe. The humming would go away if I turned right even the slightest bit but continue no matter how far to the left I went. My problem ended up being a bad wheel bearing. 
