my friend has a 2005 forester ive been working on. every 30 to 45 seconds when its warmed up theres what seems like single missfire then it goes back to normal idle. it only does it at idle and is not throwing a cel for it. what could be causing it? it may not be a missfire but thats what it sounds like. idle idle hickup/missfire sounding thing idle idle

2005 is old enough to have 3-4 things that might cause this.

old plug, old plug wires, leaky VC gaskets, knock sensor,.....etc.

live data from the ecu might be a good place to start.