Just replaced a leaking PS pump on an 2004 STi for a friend. Followed the book to the letter, including the priming and air purging. While rotating the wheels to the left with the car on the ground, it has a bad throbbing chatter at idle, like it is stuttering. It is not noticeable while driving it. The pump is quiet, and it is not foaming or bubbling in the remote mounted tank. I spent about 10 min rotating the steering wheel from far left to right purging the air. Fluid is OEM Subaru. The reservoir has been vacuumed empty, re-filled with fluid and run for several minutes while rotating the wheels several times. The fluid looks perfectly clean. Any ideas? I am hoping that there is just some residual air in the pump.