Burning smell

I noticed a burning smell in the drum after drying a load of clothes and my clothes smelled burned. I took the lint trap apart and vacuumed inside of the casing, and the exhaust vent coming from the back of the dryer and the tubing from the dryer to outside of the house.I opened the back of the dryer to check wires, everything looked okay and no smell in the back only inside of the drum. The drum still spins and the dryer still heats up. no loud noises everything sounds smooth and no smoke noted. what can this be?