We have an '02 325. I also had your problem - could hear the pump (half a block away!) but couldn't find it.
In our rig, the foot of the bed blocks off what I thought was a "dummy" double cupboard when the slide is in. It's below and set back a few inches from the main wardrobe/drawers unit in the bedroom and the electrical power system controller is next to it. That's the LH rear corner of my rig.
I was tracking down the TV cable wiring, which appeared to go inside the compartment where the power controller was, so I did some more careful checking with the bedroom slide deployed. It turned out that the "Dummy" cupboards were real ones, but the doors had been "permanently" closed with a big woodscrew in the top corner of each.
I removed the screws, opened the cupboards and there sat the potable water tank, the pump and a tank drain valve! I also traced the coax cable for the TV and replaced it! I can now drain the potable tank without having to run the water through the faucest and into the gray water tank!
Your rig might not be exactly the same as mine, since it's longer and newer, but the noise is coming from the same area as mine did.
Frank and Eileen
No longer RVers or FR owners