Groaning/humming noise

Ford F150 Pickup. There is a very loud groaning or humming noise coming from under the back of the cab area. It's not steady, it "hums/groans" for about 3-5 seconds and then quits and then continues, while in motion forward and reverse (under 5 mph, haven't had it on the road to test at higher speeds) and in Park. Then after about 1 minute, in Park, it quits and then starts again, and so on and so on.

Anyone know what that is?

The exhaust gas pressure "puffs" to the heartbeat at the tailpipe?. Aside from the very loud deep slow heartbeat humming which is actually intermittent (when passing thru gears), when it is not "humming" it actually sounds like a constant loud deep version of a lifter knocking/ticking (inside the tailpipe as I said). I used the video on my camera to try to capture the sound but it can't be heard on camera audio (the ticking can but not the hum). Weird, considering how loud it is. I don't see where a video can be posted here anyway.

I would check the fuel pump and make sure the transmission is full of fluid.

rear brakes could be sticking too close to the drum creating a dragging noise. is there any hesitation when you try to move the truck?

rear brakes could be sticking too close to the drum creating a dragging noise. is there any hesitation when you try to move the truck?
No hesitation

do you have two drivelines with a "carrier bearring" in the center? check for excessive play.

also check where the driveline goes into the back of the trans for excessive play. you only have to push up and down on it. it should have a very slight movement. anything more is excessive and probably a problem that should be addressed. you could always put the rear of the truck on jackstands, chock block the front wheels and have someone put it in gear and look underneath it to see what is happening