The first suspect should be the transaxle starter bushing. Most starters have a boss that is part of the starter case and this supports the armature.
The VW air-cools do not have this. They rely on a bushing that is mounted in the transaxle case and this is something that your mechanic SHOULD have replaced when he rebuilt the engine.
When the bushing wears this allows the starter armature to sag a bit and it rubs against the field coils in the starter motor.
There is always the possibility the starter could be failing but you could try replacing the bushing first and see if that fixes it.
I don’t know if you will attempt this repair yourself or not but it’s not difficult to do. Maybe unusual if you’re not familiar with it. The repair goes like this if you do it yourself (after procuring a new bushing).
Disconnect battery; remove starter.
Use a thread tap of a size that will fit snugly into the new bushing and thread this tap into the old bushing in the transaxle case(you’ll need a light and have to stretch to eyeball the bushing). Turn the tap into the old bushing until it bottoms out and keep turning until it forces the bushing out. The bushing is a “blind bushing” which means it is in a dead-end hole and there is only one way out.
The new bushing should be of a material called Oillite. This is a porous bronze type material and the next step is critical. Place the bushing on a thumb, fill it will heavy motor oil, and place the other thumb on top of the oil filled bushing. Apply pressure for a minute or so until you see oil ooze out of the pores on the outside of the bushing.
Use a bolt with a nut/washer as a driver and tap the new bushing into place. Reinstall starter and see if that fixes it up.
This is a common problem with air-cools and I’ve done a bunch of them for what it’s worth.