Check the door lock line with a mighty-vac. If it doesn't hold vacuum, go looking for the vacuum leak. The 124/126 lock circuit has a switchover valve inside the pump to issue vacuum or pressure to the lock pods to lock or unlock the doors. If you have a leaking vacuum pod or a broken vacuum line somewhere, it would make sense for the pump to run until it times out. They normally make a soft whine/whirr noise when they run, so unless it's exceptionally loud it's unlikely to be a bad pump.

__________________
Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]