Did this just start happening and didn't happen before? I'm asking because, at least on my 94 530i, is that when you lock the door on the driver's side, there is an extra grinding sound for half a second which I think is normal and it is the gear that locks the deadbolt into place and I think I remember my older 535i making the same noise.
If you lock the doors from the inside, that is, when you are inside the car, it shouldn't make the noise though, because that does not activate the deadbolt. Press the locks down when you are inside the car and see if it makes the same noise and if it does, then I might worry about pulling the door trim.
That's my two cents for what it's worth. Before you start taking the panel off, I'd make sure it's not just the deadbolt doing what it's supposed to do.