C13 Acert Help

I have been scouring this and other forums for the past month or so to try to figure out what is wrong with my truck. I have a KCB C13 that has a slight knock/rattle when I'm into the fuel. The side effect is that my fuel mileage is down about 1 mpg. I have had it to 2 different Cat shops and neither one can figure it out. They both agree that something is wrong but can't figure out what it is. It has been on the dyno a few times and they say everything checks out. I origionally thought it was an injector on the way out but when we cut them out it didn't seem to make the noise go away. Overhead was just ran and didn't help. Shortly after that I had a code some on for an IVA. Shortly after that the engine developed a tick. That ended up being a broken spring. Then I got another code and for an IVA, so they replaced another pair of actuators. When the mileage started to drop I put on the Donaldson mufflers. It continued to drop. May have made it worse. Also it gets worse mileage with the fuel temp wire. I run a dedicated run so the fuel numbers are very accurate. Oil samples are good too. I'm at a loss. I'm wasting a ton of money on fuel. Can any of you guys help me out?

 

I'll give it another look. The noise almost sounds like a rattle, but it is coming from the engine. You have to be into the fuel and it is most noticible at low rpms before the turbo spools up. Once the turbo spools it drownds out the noise. You can still hear it faintly though.

 

How bad would the leak have to be to cause a full 1 mpg drop?

 

I took a good look for an exhaust look. Couldn't find anything. Wouldn't the lifters make noise all the time? It only will do it when I'm into the fuel going down the road.