Kenmore 80 series top loader knocking loudly on spin cycle. Definitely a mechanical noise, not out of balance condition. Not affected by load. Started suddenly. Knocking is worst at beginning of spin cycle and decreases in intensity slightly as tub gets up to speed. Frequency is about 2 cycles per second and it does not vary with tub speed. Acceleration of tub is very slow and it is quite easy to stop the tub by hand when it is spinning empty. Wash and rinse cycles unaffected. I'm guessing "clutch", but would appreciate any words of wisdom.