That sounds like the startup airpump noise my '93 M104 makes. You will know when it goes south , screech, scrunch, bad bearings, horrible sound.
My '92 did go south suddenly, while out of town. I simply unplugged it.

I eventually replaced it with a re-built unit from CVJ Axle in Denver, about
$400, if I remember correctly. There is also www.mbpumps.com, which I just learned about on this forum. Airpump for around $200 - 250. They also sell just the bearing kit.

As previously mentioned, if you replace the pump, replace the manifold valve.

Steve