Pump drains water but sounds like it is cavitating at the end of the cycle

Have been getting the suds and F21 error. Replaced the pump and pressure switch 6 months ago & worked fine for awhile. Then the pump started making a grinding sound when draining & the F21 error came back. Took apart the drain line again. No blockage, but the ball valve was pretty gummed up, so took that off and cleaned it, but no luck. The pump sounds fine when first draining and empties the washer, but then keeps going when it appears that there is no water left and it sounds like it is cavitating. Is this due to a bad pressure switch, even though it was recently replaced?