I just noticed this yesterday when my stepdad was backing out my car. The car was going on a decline in reverse and he pressed on the brake lightly to peak out into out alley and a very faint "BING" noise kinda like a spring poping or something like that happened only once. Now ive heard of people saying the brake booster makes a slight noise on the newer b6 and b7 but is that what im hearing? I know it came from the brake system cause he said he felt it "grab" as well. Any suggestions?