Hi usually but not always that problem is due to brush wear and bushings need to be cleaned and polished! you have to take the face plate off with the brush holder and disassemble the side plate to have access to the commutator as the commutator gets scratched thru time and if you get some very fine emery paper you can polish the face of the commutator and will look like a Mirror, also brush springs lose their tension and then don't put enough pressure on the brush which can cause brush noise like you described! best to do it purchase new brushes and springs and polish commutator as was described and probably run like new! other issues can be gearing noise for lack of quality grease lubrication! red thick and tackey is a great automotive grease you can purchase anywhere. if you don't was a large container like for automotive you can purchase smaller 4 pack from Harbor Freight! p.s. Lionel post war whistle tender in steam locomotives had same issue and can be repaired same as above!
if you need parts check out below
Welcome to The Train Tender - Internet Reference (ttender.com)
Alan
