I have posted before about a possible clutch issue as my engine rpms surge mainly in 5th or 6th gears under medium to heavy acceleration. This does not happen all of the time. I have really noticed more when i am getting on the highway or merging from one highway to the next. If I take off from a stand still and ride it pretty hard it doesnt happen. My new problem and the reason for this post is a popping noise from the rear end. It sounds like something is breaking or metal is slapping when I take off. It doesnt matter forward or reverse. I also get a very funny noise (almost like a whistle) from the rear end when I let the engine slow me down instead of the brakes.
All help is appreciated.
Thank you

 

Go under there and check your U-joints. Wiggle the driveshaft back and forth and see if you can discover something loose.

 

the surge in 5th and 6th gears definitely sounds like a dying clutch tho

 

The surge is probably a clutch, mine just went out at 15K miles on my 6.7, and it was doing the same thing, except for the noise in your rear end.