Has to be something a long the lines of ice melting off bottom engine while driving and when you stop it drips on some of the suspension/drivetrain and freezes. I never really heard anything like that, except making turns which is the ESP, mainly cause of my exhaust being loud. I always cleaned underneath it though like every 2-3 days, max! Id say everyone take it to the wash and spray the underside thoroughly and see what happens. Salt is some nasty **** especially mixed with snow, ice, dirt, oil etc..