I personally like the growl that my '35 gives off. It drives the newbies with their high dollar Genesis locos nuts. I myself haven't tried this one but here's an oldtimer's cure I remember from some years back, when Athearn was still affordable.
-Disassemble the locomtive's geartrain carefully, noting how they fit.
-Check for and deburr all gears, bearings and axles. Clean all parts.
-Reassemble and lubricate the gears and gear tower with --drum roll please-- Pearl Drops Tooth Polish or something similar. Do not put the body back on at this time.
-Prop it up so that the wheels roll freely and apply power to the mechanism, allowing it to run forward and backwards for about an hour in each direction. Check the motor for any heat build-up periodically.
-Disassemble the gear train again, washing everything with warm, soapy water. Rinse and allow to air dry, except for the sintered iron wheels. Towel dry them and then coat them with a light oil to prevent rusting.
-Reassemble, lubricating the gears sparingly with a light grease and an appropriate oil on the axles and bearings.