2004 Honda Odyssey
When traveling at speeds above 45MPH there is a strange noise coming from the back of the vehicle. It sounds like a vibration sound that is somewhat hollow. It happens when I go over a rough spot, which is everywhere in PA.
Any ideas?
It could be something vibrating at resonance or rattling (the possibilities are endless, and mostly under the vehicle) or it could be a rear window not fully closed.
Is it possible to have someone else drive and you sit in back and listen?
A hollow noise could be something exhaust. If you can’t wiggle something while you’re under the car testing then it may be inside a muffler or cat. I had a strange noise once that turned out to be the spare tire not winched up tight enough, but it wouldn’t flunk the first few wiggle tests because it wasn’t on my first list of possibilities.
My wife was able to listen while I drove. She said it was definitely from the back.
I will check the muffler and exhaust pipe to ensure a tight fit and tie-up. I was actually thinking it could be the shocks, but the vehicle is only 5 years old and only 45K miles.
Does it change or even briefly disappear when you make turns? If so, suspect a wheel bearing problem - though as mountainbike said there are a lot of possibilities. You might want to have some trusted shop check the wheel bearings anyway.
Don’t discount the possibility of a worn upper rear shock mount. For me the first thing that comes to mind concerning strange noises like you describe is a tire that has a lump on it.
I only notice the sound when traveling above 45mph and my turns are usually below that speed 
I will investigate the wheel bearings - thanks!