75% of the time when I shift it into reverse I get this rumble type vibration/drone noise from under the passenger side cab. I checked my 4" exhaust and that is pretty much clear of touching anything. My motor mounts are 3 years new. Shock mounts, stabilizer mounts, etc all tight. I know it could be a lot of other things and this is usually tough to diagnose thru advice. I am just curious if anyone by chance has had this and found the cause. Thanks a plenty

 

Had the exact same issue. My 4" exhaust was hitting the crossmember where the half u shape is formed in it.

Try this....TEMPORARILY shove a wooden shim between the pipe and crossmember. Start truck and shift into reverse. If its gone, you got it. If not, PULL THE SHIM OUT.. Either way pull the shim unless you like burning wood under your truck. Adjust the system so it doesn't hit anymore

Next is the down pipe.
Directly where the 4" downpipe intersects the passenger side frame is where it was hitting as well. If you look through the wheel well after lifting or removing the rubber shield, the pipe in my truck was touching the upper frame rail lip. If so, there are scrape marks on it from touching.

How to fix ? Since i couldn't adjust my pipe location , as i used the guillotine clamps, i took my bernsamatic torch, heated the pipe just below red, and with a large hunk of steel, i indented the pipe where it intersected the rail.

Been good ever since

 

T- thanks. Will look into it definately