I had a issue like this a few years back after I install new front Hawk brake pads on my GLI. I racked my brain on the cause and cure. I only heard the sound when I hit bumps but could not be heard when I applied the brakes. Turned out that the brake pad shim either fell out or was not installed and the pads were rattling inside the caliper when I hit bumps and was not on the brakes. Thats **** drove me NUTS!!!