Front hydra pump knock on 4250

I have a JD 4250 it sounds like a knock coming from the front pump at low RPM's when you get above 12-1300 it goes away. I've checked hydra fluid, the front coupler for wear they seem to be ok. What else can/should I be looking for? Thanks
 
I know you said you checked the coupler, but did you check the pump shaft and yoke connection for slop? If the yoke gets loose on the splined shaft of the pump, that can cause a knocking sound as well. My 4020 was doing this. If you turned the steering wheel, the sound would go away until you stopped turning the wheel. Found the bolts holding the coupler on the shaft were a little loose. Tightened them up and the noise went away.
 
I would think it would be cavitation due to lack of flow from transmission if under load, if not under load, something in coupling or splines is loose.
 
An internal hydraulic leak from a remote valve, rockshaft valve or other function will barely stroke the pump at low rpm's causing the knock. When you rev the engine up the extra flow fully charges the pump and the knock goes away.
 

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