Been reading a bunch of forums about the issue I'm having but all are from a few years ago so wanted to find out new info. I have 2008 4x4 with the 5.4, about 6 months ago I got the grinding noise and replaced the drivers side axle and actuator. Problem was solved. Now about 2 weeks ago I had a 6" lift done on the truck, immediately had a very faint grind and was told it was normal. Presently it sounds just like when I had the actuator replaced. Shop says both iwes are fine as well as the lines. Only makes the noise during acceleration and in 2wd. Cant tell if it's there at high speeds from the tire noise on the road. Try changing the check valve and see or anyone have a better solution.

 

You are losing vacuum. When you accelerate, there's not enough vacuum to keep the IWE's disengaged. Check the IWE vacuum system, including check valves and accumulator (vacuum box).

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