Subaru Problems? Ask a Mechanic for Answers ASAP
Do you feel the vibration in the steering wheel or in the brake pedal?
Try to apply the emergency brake, with foot barely on brake pedal and see if it still vibrates. Tell me what happens.
Its sounds like the ideal is to low, the throttle plates get plugged up with carbon and will lower the idle slightly, so the throttle plate needs to be cleaned.
A fuel induction cleaning will not clean the throttle plates, if the throttle plates have not been cleaned, it should be the next thing done.
Has any other work been done to the vehicle? Axel shaft replacement, flywheel replacement, list any types of repair the vehicle may have had before this problem started.