During the wash cycles, there's enough torque for the rotor splines to skip causing the whining sound. During the tumble dry, the rotor turns slower and thus is able to turn the drum with little effort.
Usually the reason for this problem is because the bolt loosens up and allows some play in the splines causing the rotor to skip when enough play has worn out the splines. The next time you see this, check the bolt first. You will find that the bolt has backed off from the rotor. I see this all the time for this problem.
That's my interpretation.