Hello Dean,

Out of my experience, it can be one of two things or both:

1- I assume you are using OEM pads. When you replaced the rear ones, did you keep the cut out shims? Are they properly oriented?

2- as the rotors wear, they develop "lips" on the outer edge. if that is the case with your rotors, particularly the front ones, sometimes during severe braking, the pads will move slightly as if they were rotating inside the cliper housing. This movement is very small, but enough to cause the side of the pads to scratch against the "lips". You will feel it as a muffled low frequency grinding coming from the front wheels. If that is you case, I suggest you grind of the corners of your front pads (carefully), so that they stay away from touching the lips on the rotor edges.

On a second thought, make sure that the allen screws (holding the calipers) that you took out when you replaced the front pads are properly tightened. If they aren't, you are playing with devil himself.

These volvo brakes are quite temperamental. They hate anything other than OEM pads. Particularly the ones that promisse to be dust free... These will cause all sorts of sounds and vibrations.

Good Luck

Icelandic