For the most part, the motor assembly is impractical to service. I took one apart sorta by accident (started removing the wrong part...once I started I decided to continue). The gear assembly is pretty much sealed up. Once the motor section is removed, you may be able to spray some lithium grease or some other lubricant in there. You may even able to pry it apart. I decided not to try.
The motor itself is the standard small DC motor, with tiny brushes and even tinier springs. I had a heck of a time keeping track of where they were. The springs for the brushes are the type that look like an open safety pin. Standard "lifetime lubricated" bushings are there too. Those could take a drop of oil in them, but if they're going, it would likely be a short-lived fix. Besides, many would follow the "if a little is good, more is better" and use too much oil which could cause problems by getting on the brushes and armature. Getting everything back together, holding the brushes out of the way with dental picks while sliding the armature back in, which wants to get grabbed by the magnets, etc....taxed my patience, which I'm in short supply of at times.
To me, it'* not worth the effort just because of noise...and if it went all together, I'd probably check out the cost of a junkyard unit.
(I know that was a bit long. My daughter tells me...."Dad, in 500 words or less, tell me....")