This. Always drove me nuts because the hills by me are pretty steep (10-12⁰) and the car would be pretty aggressive in the AM with a cold motor. I always overrode by moving the shifter over before it downshifts or forcing it to upshift then back to D on flatter roads.Are you sure it's not just in-gear and engine braking? The diesel does that on its own pretty frequently.
Does the rattle noise change with wheel speed? Or engine RPM? That would suggest something loose or broken.. or do you mean a vibration?