Re: 1997 F150 4.2 Need Help!
Several things can cause the knocking. Low octain gas. Not changing the motor oil and filter every 3-5 K miles.
Ford used hydraulic lifters for the engine oil circulation. When at low idle the oil pressure will drop causing a tapping sound. Inside the engine there is an oil pump, a lifter piston at the bottom, the lifter rod and a the rocker arm at the top. If my memory serves me well. Any of these can cause the knock. Engine rebuild or replacement may be your best bet if the gas is good and oil is clear.