You're describing what is typically referred to as exhaust drone. It is very common with 40-series. Depending on how your exhaust is configured, there is always risk of drone, but you can mostly eliminate it with quieter/better designed mufflers.
The drone comes from the exhaust creating sounds that resonate inside the cabin much like subs resonate best at a certain frequency in their box's volume.
The phenomenon is known as a "standing wave" where the length of the frequency bounces back on itself in a mirror image. You can duplicate the same thing in the shower by singing. Start sliding up in pitch and you'll get to a frequency that makes it sound like the shower walls just collapsed on your ears.