The E34 brakes work fine when on pavement. However I need to navigate down a small hill on a dirt road that is pretty rough due to rain run off. When using the brakes here I can feel through the brake pedal some sort of rumbling, almost a grinding, from the brakes. Get onto the paved main road and no more rumbles or noise at all.
Will be putting the car up on jack stands for some other unrelated work soon and will pull the wheels to get a better look.
Does the above description give any clue what I should be looking for?
If this was happening on a paved road I'd think the pads are worn and grinding into the rotors but I know that they are not.

TIA