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Hi Again
My first guess as to the cause of the grinding sound would be the fixed dog of the escapement is not fully clearing the star wheel. The most likely culprit would be the space bar linkage being out of adjustment. Check to see that the space bar, power space key and half space key (if equipped) are all sitting up level with the front edge of the keyboard bezel. Also make sure there are no bent or binding linkages between the space bar or the other key(s) and the escapement. The next thing I would look at is the universal bar or the mechanism that trips the escapement each time a letter key is pressed.
There are others on this forum with more experience than me, so I'm hoping they'll chime in with some suggestions to fix your issue. All the best,
Sky