My karcher k 2.94 makes a loud screeching noise when trigger gun is pulled and released.... any idea what this may be ???

I have a water cooled K5.140, barrel shaped karcher. Mine suddenly stopped working. Bit of a faff getting the casing apart but not beyond most competent car repairers. Once on the becnh look where the power cables enter the motor unit. Mine has a black box with clip on cover. Look between that and the metal of the motor/pump. There is a thin plastic toggle that will be stiff. WD40 on it and move the flat lever underneath it up and down with a flat screwdriver. This toggle and lever shuts off the power when the water pressure in the pump is high, so if it is stuck open you have no power to the pump. Took me less than 5 mins and a squirt of WD40.