Engine Shaking at idle. D= HELP
 

I have a 06 wrx, wagon, everything is stock. I just replaced the rear O2 sensor. and oil. when i cold start my car i hear a pump or bearing or something turn on and i hear it turning and when the car is past a certain point it stops making that noise. The shaking of the engine only starts when its all warmed up and im sitting at a light or something and i let it idle and the whole car will shake side to side and the idle will sounds a little funky and i'll have to blip the throttle a little to get the engine to idle normally again.

Things i've tried:
I've tried reseting the ECU. when i did the engine will turn over but will be very hard to catch and start. and the pump or bearing noise will come back and the idle will be very rough and will rock the car side to side. then the check engine light will return and the idle will go back to being smooth. Any ideas? I haven't gotten my 60k tune up and its about 71k. think getting a tune up will help?

It sounds like your secondary air pump or solenoids may be going out. I would recommend starting by reading the CEL codes, doing a comp/leakdown and changing the plugs, and most likely deleting the secondary air pump.

If the air pump solenoids fail and stick open, your a/f ratio can get thrown off drastically and cause the issues you describe. Fouled plugs could also be causing a missfire as you describe.

Thanks
-- Ed

does it feel like its gonna die?
and shudders really hard?