The input shaft bearing is notorious for going bad. And it could be the noise you describe. I have also had wheel bearing that you could not test bad. The didn't feel gritty they felt tight they he'd no play or run out but yet under load they were bad that is the problem. You can test them under load. I tried o read all the posts but might have missed it does cornering change the noise if so probably wheel bearing. If not input shaft bearing. Here is a link
DIY: Tranny removal and Input Shaft Bearing change - D-series.org