Why does your 1996 Volvo sound like driving on a washboard when you brake at fory miles per hour?

I have had that symptom several times. It appears to be associated with wear of the rotor, which seems to happen "normally". If you feel the rotor, there is a rough edge around the edge, and I figure the side of the brake pad rubs that ridge and gives you that effect.

In any case, replacing the rotor does the trick. One shop told me that they replace the rotor whenever the pads are replaced.

A friend reports that the same symptom in his BMW 3 series was cured by lubing the pins on which the pad moves in and out on, I con't confirm that.