Faint humming sound after unlocking doors

[jbach36]

Recently after I unlock any door, there's a faint humming sound for maybe 30 seconds and then it stops. I'm assuming this is a door lock motor of some type. I had my vacuum pump replaced last year so I would assume that is out of the equation.

I don't think it's the antenna mast motor because I don't hear it on that side per se but it's possible. (Plus I lubed my antenna so it would open/close easier without sticking).

Any ideas?

1991 300d, 228k

[Diseasel300]

There's a vacuum pump for the door locks. It should shut off automatically as soon as vacuum is built in the lock system. Running for a full 30 seconds sounds like there's a leak somewhere and the pump is timing out. In W126 chassis the pump is in the spare tire well. Not sure on W124, it may be under the rear seat.

[dieselbenz1]

The door lock /unlock pump is under rear passenger seat in a w124.

[jbach36]

Once I get to the pump, what then? I can try to find vac leaks but as far as the pump itself, do I lube it or fix it somehow, or do you just replace them?

[Diseasel300]

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.