The truck is 2018 fx4 5.0 10 speed. I bought new in December.It's not a knock noise,but it sounds like a small backfire after accelerating up to about 30-40 mph and you let off. It began at about 3700 miles on it. I took it in at 4200 miles. Dealership did the tsb 18-2354,which changed the transmission shift points some. I checked my oil afterwards and it was low. I went back and asked that according to their service sheet all fluids were checked..??? So it was still making the sound and they kept it for a week. So the oil and filter was changed lDoing an consumption check now. I was told that the sound is a characteristic of those like vehicles. This is coming from Ford and the service manager. No the truck runs great and is not knocking. But the noise is what I am not sure of. I'm going to wait a while and will take it to another dealership to see what they think..I think it's a fuel issue,as in a injector staying open too long. It only does after deceleration or gear shift. There is no jerking. Sounds like a hand clap or a pop in the catylitics..

. I run in normal mode. The sound is mostly heard at lower gears at higher rpms. After gaining speed and then letting off. There is a procedure that Ford has to go through,but it involves taking the intake off and checking some sort of gate lever. Also Ford has just put out a new TSB for oil consumption on March 1 for the 2018 5.0 liters. It starts with a new PVC valve. Then monitoring the oil levels. The end result could be a long block change out..

I mentioned something about the bores to the dealers shop foreman. He said that there are liners, but the rings aren't seating well. Thus that could explain the oil consumption. I am down some in a 1000 miles. Getting the PVC renewed today..