okay, i have a 2006 cr-v. just recently hit the 20k mark. I have had a howling/drowning/humming noise coming from what sounds like underneath the car and sometimes even from the rear but i took a closer look with 3 certified honda technicians and we came to a conclusion! the noise was coming from the exhaust! so if you are having the same noise..there is nothing wronge with your car it was just a poorly engineered exhaust system that has a very extreme resonance at different rpms and engine loads..might i add, the way to know if your have this same problem is..place your car in neutral rev the engines to just below 2,000 rpms and hold and or you may have to go higher to around 3-4 mine does it all around. but you should be able to hear the noise if not then you may actually have a different problem/ noise! just a tip. hope it helps someone!

If your are having this noise you cannot fix it, just turn the radio up louder and ignore it..

~ Brett

Before accepting that explanation, you might want to take a look at TSB 06-044 (Drone, Moan, or Buzz at 1,800 and 3,600 RPM)
http://www.alldata.com/tsb/Honda/1153292400000_1154934000000_06-044/6044.html