As Ed said check the connecting arm to see if the ends are loose on the rod. The rod ends will pivot on the ball ends and should do so rather freely. You should put a couple of drops of 3 in one oil on the pivot on the rod ends monthly. Also there is a grease zerk on the paddle feeder shaft give it a little grease twice per season. The next step would be to remove the all the coal from the hopper and pull the paddle out and clean up the shaft as some fine may have gotten into the bushing if there wasn't enough grease to seal it out. Usually that type of motor will groan or whine loudly when they are on there last legs so I would think that the motor is probably still ok.
I have noticed that if I speed the feed rate up the noise stops. It is only on very low burn rates like 1 or below. I did grease the fitting and lube the connecting arm ball joints. I will keep a close eye on it. Thanks again. The help is great.