what I do know is that there IS a conversion pulley for the motor that makes the pump go faster for say use in a basement when it has to pump higher and you may have this one installed, there's a parts store near me that has them, it appears to have the drive pulley and the pump pulley the same size, which makes the water pump out faster, about 35/40 seconds compared to 55 normally, no anti-siphon installed, an 8 foot hose now has to be strapped down in the tub because it now blast out like a rocket
you may need a plumber, but first try pouring some lye/acid down the drain, and flush with hot water you have heated on the stove to boiling, run a garden hose down the drain to flush, or even run a snake down there before calling a plumber...you may just have detergent/softner/lint buildup
just a thought!