Anyone know why Whirlpool washer machine would make knock sound while spinning. Only while spinning not agitating. Must not be draining properly clothes are still damp.

Ultimate Care II Heavy Duty Super Capacity Plus.

Make sure first you do not have a badly out of balance load. The knocking usually is the tub hitting the sides.
You might try to spin it with just a load of water. If it is OK with just water, and you cannot correct it with clothes - there are springs that hold the tub stable to the base and one may have come loose.

Neos is correct in what he said. One other thing you may want to check and that is how well the pump is draining the water. If you find that the stream coming from the drain hose is not very strong then there may be a small piece of clothing that is blocking the water from getting into your pump. It doesn't happen often but over the years of servicing washers I have pulled many a small sock out from between the inner and outer tubs.