I have a 2005 Trailblazer 4 door LS. When I step on the brake pedal the pedal vibrates and a buzzing noise is heard almost like groaning. The brakes feel soft for a few seconds and then the noise and vibration stops and the brake pedal feels hard as you complete the stop. It almost feels like air bubbles in the brake lines are being released. This happens all the time and at various speeds. Last night I was testing it to get a better description for this forum of the issue. I would be doing 10 MPH heading to a red light on a perfectly flat, brand new paved street, no bumps or rough road conditions. I lightly applied the brakes and the vibration started and then ended when I was at about 5 mph till stopped. This happens every time. Next I pumped the brakes and pressed them to the floor multiple times. This issue of the vibration and basically the groaning noise stopped for a few stops but then returned. I thought it might by the ABS system so I pulled the fuse on it. The ABS light was lite (as expected). I took the truck for a spin and still had the same issue. Brake pads and rotors were replaced 5K ago but this was occuring long before the brake job. The brake job did reduce this issue a bit as far as the length of the vibrations and the frequency (almost stopped) but the frequency has now increased. Anyone know what could be causing this, is there anyway to diagnose this issue better and fix it?