Driving my wife's 2002 Forester, I noticed today something I don't remember seeing before.
At idle, it oscillates (with a full period somwhere in the 1-2 min range): the engine goes from nice and quiet ~ 600 RPM to noticeably rougher, almost shaking, 1000 RPM. Weird. Wife tells that the car "breathes" and she considered it normal - so I don't know for how long it's been going on. Could easily be few thousands miles and over 6 months.
The driving part feel normal - no oscillations for no obvious reasons, reasonably smooth and does not feel underpowered.
Any guesses as to what it might be caused by, how bad it can possibly be for the car's health and what can be done to fix it (if fixing such a thing makes any sense)?
Thanks!
Dima