I just had the trunk battery replaced a few months ago, as it wasn't holding the charge anymore. In about two months, that battery also started dying on me. I called mercedes and they came out and checked the battery and said nothing was wrong with the battery, and something might be pulling a charge out of it. Even with a trickle charger hooked up now, when I disconnect the charger, the battery is dead within a day.

Then I heard my car in the garage one night making a noise. About once every hour, I hear it make a rather loud "grinding noise", which sounds like some sort of pump, kind of sounds like it might be coming from the front right engine bay. I've heard it before when I have started the car, maybe it's some brake booster? Anyhow, it's doing it on its own about once an hour, and I have to wonder if that's what's causing my battery to discharge.

Any thoughts on what is it, and why it might be happening?

Suspension pump maybe?

Sounds like the ABC pump.

You can try pulling the fuse when you hear the grinding/pump noise to check.

That is what I was going to say! The best thing would be to have the car checked at the dealer.

Instead of going to the $tealer just buy the MB Star Compact 3.