rumble

Hi everyone. New Ram 1500 Owner here. I just traded in my Tacoma for a 2013 Longhorn edition 4x4 w/ 5.7 Hemi (56Kmiles) about two weeks ago. Everything seemed fine on the lot and driving it around. Carfax checked out, but this weekend I took it for about a 4.5hr roadtrip and noticed when I had...

So it's very slight, but noticeable and I would describe it as a slight knock/grumbling/rumble. It's one of those noises that is hard to define. It's rhythm is relevant to speed - it gets faster as the truck speeds up. My first thought is a grumbling diff. I just recently serviced the...