Power Steering Noise at ~2,000 RPM, Only When Warm
I've got a 2011 STI hatch with about 117k on it. I feel like i've always had PS noise but it seems to be worse now. Maybe three years ago i noticed that there appeared to be a slight PS leak under the car, which I determined was most likely the pinion valve seal leaking, but interestingly enough it isn't that bad and doesnt affect my fluid level at all. Recently, i upgraded all the spring clamps in the system to worm drive clamps, replaced the o-ring on the pump with an OEM o-ring (the correct part), and flushed the system and bled as best as i could.
Since doing all of this i eliminated a slight PS noise when turning just after starting the car cold. That is completely gone now, however i notice once the car is sufficiently warmed up if i rev the car up to 2,000rpm (in gear or in neutral) and just slightly turn the wheels in either direction, the pump will start to whine and will continue until about 3,000rpm, when it stops. the noise pitch increases with engine speed and i am certain it is the pump making noise. Overall, my steering feels normal although it does sometimes hesitate just slightly when i'm in that engine speed range and can hear the whine. If i hear that noise and turn off the car, checking the reservoir reveals foaming, so there is definitely air in the system coming from somewhere.
I've removed the belt from the pump and spun it by hand; feels smooth, very little play on the shaft. The pump itself (nor frankly, the entire system other than the pinion valve) does not leak (i almost never have to top off the reservoir unless i am replacing the fluid). I've considered replacing the rack or pinion valve but honestly the leak is minimal, if anything. I figured maybe the pump is bad but it doesnt leak and the shaft and bearings look to be in working order. The lines themselves are still soft (California car, bay area doesnt get too hot or cold and no snow) so I dont see a need to replace it.
I guess my question is whether air could be entering the system through that pinion valve seal. Maybe the movement of the rack is creating a vacuum that is pulling air through the valve, but I've heard that air usually enters the system through the low pressure suction side, which i've tried to remedy with new clamps and an o-ring. Any ideas?