[alia176]

I'm not convinced that your O ring is the actual issue here. It "sounds" like the PS pump itself is freaking out. If you indeed had a leak where air is going in, wouldn't there be fluid leaking out? Furthermore, if your res is sucking in air and then pumping it throughout the PS system, wouldn't you be experiencing cavitating and subsequent loss of power steering?

I believe I have the same sound you're experiencing and waiting for the PS to take a total dump. If this unit is rebuildable, then perhaps I should look into it. My only real indication was when I lost steering effort below 1500 rpm on couple of occasions. The rack has been rebuilt so that's eliminated.

Just thinking out loud...