08 Odyssey EXL 55K on it.
When I get up to 40 mph, I get a fairly loud groan or hum out of the rear end. I rotated tires and even worked in a 5th to rule out tire noise. I'm thinking wheel bearings. Is this common? Anything else to check? Are rear bearings warrantied under powertrain?

 

feel the tires and make sure there is no uneven wear. Rotate it again and see if noise is still from the back. If it is then it's not your tires

 

I'm not sure how common it is but my 06 EX-L needed a left rear wheel bearing at 130K. I just replaced both rears at the same time. It's not too difficult but I'm an automotive technician. The price of the factory bearing isn't bad and the quality is great.