I just asked a question about a leaking y pipe pass. side I think it might have been leaking and I didn't know it one day it would run great the next I would have a little vibration.

 

I changed the engine oil and filter today, didn't get Rev X yet wanted to see how it would run with clean oil. I also cleaned the EGR valve it was clogged with soot. I drove the truck after and the vibrations were a little better and could feel it had a bit more power from the EGR valve cleaning. I noticed there has been a faint whistling type sound when you ease off the accelerator and between shifts, and again this is only in the first 20 minutes of driving when the truck was cold. Possibly could it be a vaccuum leak or wastegate? Other than that the truck runs really good. I should man up and get Rev X and see what it does. Any other thoughts?

 

The whistling could very well be a cracked up-pipe. Mine used to do it too. It was an ear piercing whistle at idle, and was only there when the truck was cold. After I drove it and it warmed up, it went away... no other symptoms so to speak.