postmark2

I have had my MY 2000 CL (was pre-certified by MB) for last 4 years with no major problems. Only drop off servicing has been faulty brake switch and leaking vapor something or other in gas tank that lit the mil light. That was a year and a half ago. I just turned 68k miles and have noticed a soft ding sound when I go over bumps or sudden dip in road. Almost like the put-on-seat belt ding sound you hear on a plane. Car runs fine and I never turn the abc switch on. Has this happened to any of you? Any thoughts what the "warning" sound is? I do not have to be doing over 65mph to hear it? Thanks for responses.