my wife rear ended a truck. when she got home there wasw a knock in the motor. motor does not knock between 50 and 60 mph. never had a oil leak now i do under radiator. motor doesn't knock when rpms are low like right after it shifts gears.

 

Your truck has a oil cooler located int eh radiator. Get teh leak fixed. I would also flush the oil system. She might have knocked a piece of debris loose and it is partially plugging an oil passage.