I run BB7 road discs on my commuter. When I switched to Shimano centerlock hubs with Shimano rotors (LX level...I forget the exact model number) the rear started screaming.
I've painstakingly set them up, and re-checked the setup. It's perfect.
I've cleaned them.
The front works great, and doesn't shriek.
The rear is pretty consistent in it's protests.
Do I need different pads?