when our washing machine drains during the spin cycle water backs up into the bathroom sink which is 4 feet away. however when you use the sink faucet, water doesn’t back-up. we had a plumber snake the drain pipe to the main drain pipe of the house and the back-up still occurred. he also inserted the washing machine cold water hose directly into the machine drain pipe at full force and there was oddly enough no back-up. it only happens when the plastic drain hose from the washing machine is in the drain pipe.the plumber recommended pouring a strong residential use liquid draino type product into the drain and let it sit over night as a possible solution, assuming there is some sort of partial blockage in the main pipe which will allow lower water volumes to drain and not the heavier volume from the washing machine. FYI washing machine is 5 yrs old problem started around a year ago.