Hi Trevor,
I know why it makes noise. It pulls air in past the o-ring. I'll help you out with this. The reservior sits on top of the pump. The suction for the pump is between the reservoir and the pump. When you take off the reservoir to change the o-ring you will be looking in the pump suction.
At that joint between the pump and the reservoir, the pressure is sometimes less than atmospheric. If the o-ring leaks, air comes in from the outside since the pressure in the suction is less than atmosphere. Also the air flows easier than the fluid because air has less viscosity. However, when the pump is off the pressure is not high enough for the fluid to leak the other way. It just the height of the fluid trying to push it out, about 2.5" (60mm) of head. Not much.
I noticed on mine that the noise got worse when it was cold out. I changed the o-ring and the problem was fixed. Then after about 50k of high speed highway driving, bringing the PS pump up to 150k miles since new..... it start to make real whinny noises and to leak from around the shaft.
I put on a new one ($320) and now I have one to rebuild
in about another 100k miles or so.