I have been to 2 volvo dealerships and the second one told me this. He plugged in an electronic "keyboard" thing into the electronic part that is in the armrest compartment and duplicated the sound for me. He told me it was the self-checking/regulating mechinism that is designed to to a self check after 30 minutes of driving or so. He seemed to have a manual open that he was referring to, but when I leaned over to look at the manual, he said, "that's not the right page".