This post is fairly old but I just recently ran into a very very similar issue with my 2018 4066M. The tractor would emit a loud grinding noise when I let off of the petal. It was especially prevalent when going down a slope. My first thought was that I had developed an issue with the rear differential. I surmised that over the course of the 800 hours on the tractor that something had worn down somewhere in the rear end. It sounded just like an old farm truck with excessive backlash in the rear diff. The issue was getting progressively worse, so I was about to load her up and take her to the dealer when I noticed that the brake control arm on the right side was bent upward. It wasn't really bad, but not straight. Because this was obviously 'user error' and wouldn't be covered by the warranty, I figured I had better bend it back and re-adjust the brakes before I dropped it off at the Deere dealer. As I was backing the tractor out of the barn I noticed that the sound had magically disappeared. Like completely gone. The e-Hydro on the 4066 is really good at holding the tractor when you back off the petals (even on a steep incline), so I hardly ever use the brakes and they couldn't have been more than ⅛ in out of adjustment, but apparently that was enough to cause an issue. As a side note, the "Power Match" system had always… sucked. It kind of worked but I ended up turning it off. On a hunch, I turned it back on, and it now works perfectly. My guess is that the brakes had always been slightly out of adjustment. Whatever I hit that bent the rod really set it out of adjustment, but I never once felt the tractor being pulled the the right when driving, towing, or using the loader. I guess the moral of the story is to check your brakes every once in awhile, even if you hardly ever use them.
