W140 Vacuum Door Close Assist Pump Keeps Cycling

[Western140]

The driver's door close assist wasn't working on my 1998 S420, so my excellent independent Mercedes shop replaced the following:

1. Actuator bellows in the driver's door
2. Rebuilt door closing assist pump

Now, all of the pneumatic door closers work again, but I think there's an issue. I can hear the door closer assist pump cycling on every few minutes when I'm driving.

Let me describe exactly what's happening. When I soft close a door, I can hear the assist pump humming while it provides pressure to the actuator to close the door. That stops after a few seconds. Then, a few seconds later, I hear a second, slightly differently sounding humming from the assist pump, which I assume is the pump providing vacuum to reset the actuator. Then, as I'm driving (or even if the key is in the on position and I'm just leaning in the trunk to take a listen), I can hear the humming sound of the vacuum pump (the second sound during a door closing action) cycle on for a few seconds. And it does this intermittently while I'm driving.

I don't remember hearing this pump cycle when I was driving before. It's not supposed to do that, right? Any ideas?