Engine Knocking while parked accelerating?

Engine has a knocking sound when I'm parked and rev the engine. Quiets down to nothing as its just running. I did an oil change and put some lucas in and it quieted it down a bit but its still there. No metal was detected in the oil. I don't hear it when driving unless its just hard to hear. Any suggestions or tests I can do? Oil pressure seems fine. It does leak some oil over time. Someone said the engine might have a bad lifter vs spun rod. Or someone else listened sand said maybe its the exhaust. It sounds like its right in the middle of the driver seat and passenger. Back of the engine. Ford e-350 120,000k miles on it. Everything is in good shape other than the body and a speedometer needle going crazy. Tons of rust underneath. Thanks.

Added (1). Also has a hard time when i punch the gas while driving up hill.

Camshafts turn at 1/2 crankshaft speed, so lifter taps are slower and actually make a tap, tap, tap sound and con rod bearings make more of a rattle usually, at start-up and when the engine is decelerating from reving in neutral.Crank problems/sounds vary under load and cam/lifter problems usually don't change other than the speed of the tap.Year and engine size/type would help in the future.

A hydraulic lifter losing pressure usually makes a higher pitched tapping noise, unless it completely collapses. I've seen/heard older Ford catalytic converts break down internally, they have a honeycomb designed catalyst filter inside, and the loose pieces bouncing around inside can sound like a rod bearing knock. Loose torque converter to flex plate bolts/nuts will make you swear your lower end is shot but if you put in an oil additive and the noise quieted down then it's emitting from inside the engine. If you don't have access to an engine balancer then at idle, with the noise audible, using insulted pliers, pull and replace each plug wire one at a time listening for the noise to diminish each time. Removing the spark takes the force of combustion off of the rod bearing. OK I didn't see the update until after I posted, now I'm leaning more towards a catalytic converter breaking up, making noise and restricting exhaust flow.

You may have lifter noise, but not a spun rod bearing. It could still be bad, like overly worn, just not spun. Try using a heavier oil, like 15-w40. That may quiet things down a bit.

Hi you know full well what i will say so why ask. Old truck full of rust and now the engine is knocking time to be thinking about a newer one.