As the title suggests, the noise is audible inside the cabin (refer to the attached short clip, recorded with the phone placed near the comand unit, engine off, fan off). This is a new pump, Dolz, replaced from the original Bosch one along with the hoses and the heater solenoid. The pump runs most of the time while climate control is on. It is well secured, the humming turns into buzzing when pressing it against the thin wall in the wiper motor area. I suspect that the grommet isn’t dampening the noise enough, but the pump itself is far from quiet.

The old Bosch made the same noise, hence why I replaced it thinking the bearings are on their way out.

The noise is bothersome. Not sure if this normal? I know the M273 and other engines have a similar pimp but with more outlets.

For ref, pump is A2218350164, grommet A0009980705

Test both pumps off the car. From a bucket. If they sound different off the car when pushing fluid then that may be an indication of a restriction in the cooling system or heater.

If you've recently changed the coolant and are using a diffaerent coolant from the MB specific, then you'll need to flush the entire system thoroughly as mixing some compounds can cause gelling of the fluid. Same applies to screen wash.

I remember replacing mine. I have zero noise. I would remove and confirm you could run a water house through the lines with no restrictions. If the pu.p itself is noisy, it's probably defective

OP be sure to check that the pump is mounted such that it doesn't touch the car. Should be hanging in fairly soft rubber all around it. Look for a hose clamp on a hose in close proximity to the pump that may be against the car. The pump makes some vibration but it should get easily absorbed in the hoses and mount for the pump. If the pump itsself is growling try another. A 'new' pump doesn't guarantee one that is in good working order.

Is it making heat when engine is off and set for 'rest?'

Those pumps are notoriously loud, especially aftermarket ones. The Dolz units are often noisier than OEM. It's likely normal operation, just more resonant in your cabin. Check if it's touching any sheet metal directly. Try a genuine Mercedes grommet if you used a generic one. If it's still too loud, the only real fix is going back to a genuine Bosch pump, but even they hum.

I noticed that on the W211 (it sits by the passenger headlight, very easy to access) where the Bosch emits more of a high pitched whine rather than the hum of the Dolz. I recall comparing both (the old and the Dolz) and they were identical in terms of weight, the Dolz spinning with less resistance (and noise). Nevertheless, the hum is there. I didn't replace the grommet as I only really came across it after having removed the old pump. It's such a thin piece of rubber and MB charges nearly $40 for a special order that's set to take 1 month. It wasn't an option to have it replaced on logistics alone. It was in an ok condition, cleaned it, then drenched it in a silicone-based spray a couple times to ensure it won't dry out.

Installing the pump was a breeze (not). Maneuvering the damn thing into that tiny spot is very challenging. If I didn't have the front end almost completely out it would have been even worse. I stood between the front bumper and the lower engine support bracket, had a thick blanket over the engine to be able to work without breaking my back. The pump is affixed using two M5 or M6 nuts that are only accessible by bending the engine heat shield in a corner. Sliding the pump into place was akin to someone with all their limbs in cast trying to acrobat. I was worried of damaging something in the process. Essentially: pump goes in vertically (intake is up), then turns toward back of the car in almost 90 degrees before sliding it all the way towards the brake master. Oh and needed to remove the rain drain (which was also a bloody ***** to put back in) to have just that little bit more space to be able to plug in the pump. The grommet would need to be put in onto the pump, before pushing the pump inlet into that oddly shaped hole, THEN sliding the grommet into place. The last challenge was getting the hose going into the cabin installed. Needed two types of clamp pliers to do it effectively.

MB fails miserably at designing for servicing (and dismantling). Rant over.

I had the same problem few months ago. The heat pump would buzz and I knew it would die when it got cold.
I got a MB pump for 250 or 300 and charged it myself.
it was difficult, don’t know how I did it but I changed it. Very little space to take it out and even harder to put the new one in.
Tip. I broke off the plastic pump nozzle to be able to take it out.
To put the new one in, the only way I could do it was by removing the rubber sleeve/ mount around the new pump. Installed the pump, cut the rubber bracket in one place and then somehow slip it on the new pump. The mounting 2 bolts didn’t align but by that time I didn’t care. Hands were cut in various places and I wanted to set this whole car on fire.
My lower back hurt badly for a week.

Noise is gone and heat works. Good luck.