I think the warning buzzer is built into the dash gauge assembly, so you may have a loose air connection there. There have been other reports of bad connections there. As I understand it,though, the alarms triggered by the gauge action, so anything that makes the needle on the gauge go down will trigger the alarm.
Contact Freightliner Customer Service for chassis air system info and have the last 6 digits of the VIN handy. 1-800-FTL-HELP