My Aztec, same year and motor as SM, makes the same sound. Was concerned at first, since none of my other wms EM's do this, but forgot about it since I have no issues.

I own Space Mission as well as a bunch of other em games from all the manufacturers. It makes no different noise than any other machine except the swing target motor .
Do you own other ems?
If the score motor is making a noise at the end of the cycle it may be a dry bearing.
I had an unusual whine on the running man motor on my Upper Deck. I tore it apart and lubed the motor bearings and have not heard from it since.

If I had to guess, it's probably the spindle connected to the clutch in the motor that's dragging or needs to be lubed. My Captain Fantastic did that for a while. Basically, when the motor is engaged, the small rotor is "sucked" into the stator housing through an electromagnetic force, and the friction it makes on the inner plate is what drives the rest of the gearbox. When the motor is disengaged, the magnetic field collapses, and a small spring forces the rotor back out of the housing.

With the game off, you should be able to press on the small "drum" on the motor, and should be kinda springy, but with smooth action, and no sticking. I fixed my issue by pulling the housing apart, and cleaning the gunk and stale grease out with some alcohol. I think in some manuals, it'll specify the motor's lubed parts. I don't recall if I put any grease on the spindle part--if I did, it was a *VERY* small amount of teflon lube gel. Either way, it corrected the noisy motor, and it's been buttery smooth since.

Just make sure you keep track of all the pieces/parts and their orientation. I recall having a bit of trouble getting things back together again. But I was younger, stupider, and less experienced then, so it might be a cakewalk for you guys.

It would be inside the motor under the cover on the left. Here’s a couple pictures from the web I found that hopefully show what I’m trying to explain.

This first picture shows where the “drum” would be inside the motor. Circled in red. You might need to use a small flat head screw driver to test it.

8FC5718B-B81B-4CC6-8C42-9A086BA30508 (resized).jpeg

Once you remove the two bolts, you can see the whole mechanism. Here it is disassembled:

A9424443-2176-4AC7-9938-6EC46D58DF8C (resized).jpeg

Every motor is slight different from manufacture to manufacturer, but they all operate along the same principle.