you may have sand in a motor. Sand has a knack for getting into places it isnt wanted on moving parts. May want to try taking them apart and giving them a good cleaning. pop off the e clips on the ends of the motor shafts, get a can of compressed air (this stuff you get from office supply stores for clean keyboards) its moisture free so its perfict for cleaning electronics. the tube that they come with also is great for getting into that tiny space between the upper and lower bearings of most motors.