Steering vibration

Most likely your ps pump's top-mounted intake pipe has a worn or cracked inner o-ring, allowing intermittent cavitation of air into the pump. This is a VERY frequent problem, and is easily remedied by prying out the o-ring from the pump's top orifice and chasing a slightly THICKER one at any hardware store. Bring the plastic connector for precise fit if you want, but don't get an 0-ring that's too wide for the pump's orifice. You'll note that the old ring has flattened or been malformed into a triangular cross-section from age and pressure. The fact that Subaru uses only ONE offset 10mm bolt to hold the connector onto the pump is the reason that an old hardened o-ring can leak when the connector-to- pump geometry is canted by the offset bolt. A fatter o-ring should permanently solve your problem. Note that if unattended to the cavitation will proceed to be chronic, causing foaming of the fluid, overflowing your PS fluid, and complete loss of linear steering! BE CAREFUL! Once you've finished replacing the o-ring, refill the reservoir, start the motor, and cycle the steering wheel slowly from end to end a couple of times until smooth, adding fluid to 2/3rds full as necessary. Repeat if the system sucked fluid too quickly while being "exercised" and its level dropped below the reservoir's bottom hole, reintroducing cavitation! So having someone saw the wheel back and forth (again, FULLY) to the endpoints as you replenish PS fluid (use a stop-leak version to help the old steering rack's seals, too), is a best way to finish up. Have fun...it's an easy DIY-er in less than 15 minutes. Ern TSG/B