I wouldn't suggest the bronze gears. If you had an obstruction while closing, the stock gears will give, the bronze ones most likely wouldn't. Unlikely it would happen, but I wouldn't want my finger to be caught. GM used a grease in the stock configuration that breaks down over time and turns to powder. (this is what the bushings replace in the rebuild kits) Yours is probabally starting. The motor is continuing to run on shut down 'cos it doesn't know it has reached the stop point.