I have an old 91 Minn Kota that was making a strange grinding noise when running in the forward gears, but the reverse gears were very quiet. I thought I would take it apart and lube it up real nice to see if that helped. If you don't know what you are doing, don't try to take it apart. I could NOT get it back together. The spring loaded brushes are almost impossible to hold in place while putting it back together.
I took it to an authorized Minn Kota repair shop and the guy replaced the brushes, polished everything up, replaced the seals, lubed it really well, and cleaned it up inside and out. It cost almost as much as the cheapest new Minn Kota trolling motor but it sounds and runs like news.