hi guys i have a 04 s60 138k has been running good up until a week ago iat idel the gage says 650 rpm but i noticede the heradlights dim a little and can feel a small repeating vibration...my question is 650 rpms normal? when i press the throttle a little bit to say 750 rpms the vibration goes away and the lights do not dim. any way to crank up the idle a little bit. any advice would be greatly appreciated