That may just be extra boost backing out the intake. If you give it throttle and back off real quick the power hast to go someware so it goes back out the intake mine does it all the time in traffic. If that is what your truck is doing

 

Yeah it's called turbo fart. There is no blow off valve or wastegate on these, so this is how they expel built up boost when you let off the skinny pedal, just goes back out the intake. It was designed to do that.

 

The truck always did it , but now its real bad. the cummins doesnt do it at all.

 

Because the cummins is a different motor and doesn't work like that...

If you think there is a legitimate problem the only way to find out is get the turbo off of it and inspect it. Is it still boosting normal? Any smoke?

 

It still builds boost, but its real lazy.

 

Clean the EGR valve. When you said it was getting lazy, that tells me the it's time for a cleaning. When one sticks open the truck will just respond real slow.