Back in my drag racing days I ran into this a lot. Most guys that bought an enclosed "car trailer" just assumed the door would support the weight of about any car. The standard door will not. At least NOT FOR LONG. When I had mine, I ordered it with an optional heavier door, hinges and tensioners. On enclosed car trailers it's almost always the rear door that goes out in them. Too many flimsy ones out there. If you didn't specifically order it with upgraded door and hardware, then you got the cheapest they could get away with. When you come back to them 2 years later with a broken door, they will just tell you that it's your fault for overloading it. I ran into guys all the time on the drag circuit that had this exact thing happen to them. That door is "rated" (if you can call it that) to have ONE axle of a car on it at a time. And not a very heavy car either.
It may not explode and burst into flames the FIRST time you load a heavy car (or tractor in your case), but that doesn't mean it will take it for long. That horrible sound you're hearing is the door breaking. The frame of the door (if it has one) is cracking, as are the layers of wood. As you flex that door, it's pulling itself apart a little more each time. I would strongly encourage you to put some kind of serious bracing on the inside wall of the door. Like mount a couple of 2x8's or 2x10's along the tire track path. It would also be a good idea to rig up some kind of wood support (think like miniature wooden saw horses) for the underside of the door when it's folded down for loading.
It's only a matter of time before you break that door if you don't do something different.
My 2 cents anyway.