MikeyD 
2015 Nissan Frontier Crew Cab SL 4dr Crew Cab 4WD 5.0 ft. SB (4.0L 6cyl 5A)
What a noisy truck !!!! Till you find the problem
 Have the straps properly tied down which hold the catalytic  covers in place. When they get loose or broken sounds like the transmission or engine is grinding. Do the repair cold and you will see the rusted straps. I bought  Alum clamps for now and tied them down. No noise , purrs like a kitten... There should be a recall for the cheap straps that rusts out at 23,000 miles. They always find an item to same money on. One by the drivers side and other on passenger side. Made out of instant rust bands. Come on Nissan!!!