At about 35 to 40 mph, the 2001 Acura MDX makes a low rumble like it is going over rumble bars at the side of the highway, only not as loud or severe. The car has 137,000 miles on it. I bought it used. the dealer replaced bushings in the suspension after I brought it back. the sound is still there although quieter. Different garages has postulated: wheel bearing going out, tires are making the sound, torque converter problem. Of course it does not make the sound when mechanics drive it. Very intermittent.
Stop over to http://www.mdxers.org/forums/ and search for “rumble strip”. It’s probably your torque converter (which Acura had a lot of trouble with in these years), but it could be your EGR valve.
You might want to take a good look at your tyres. Pay special attention to the edges of the tread. If it is wavy that may be the source. It also means you likely need an alignment and/or new shocks.
I just got out of a Chrysler 300 rental that on many roads had great rumble noise. I found it would go quiet if I was out of the worn path in the road. We first started out and wifey thought we had a flat and sent the critter out to check. It was an annoyance, the car had mismatched tires at 40k, but I have to blame the tires and car design, but not a problem with the car integrity.