I've been getting P0171s (lean condition) for a while, only at very low idle though, and because it was circumstantial (and because all my other cars are in pieces and I need something to drive) I've been lazy and just drove it, thinking I'd get around to fixing it soon enough. The idle seemed a bit lower than it should be, but recently it got worse, and the car is stumbling to the point where it's throwing P301, 302, 303 and 304 (misfire for each cyl). It's fine on throttle, but the gas mileage does seem to have got even worse than usual.
Now I think about it, I'm also getting the crazy "woosh" sound, much more than it should be.. I was thinking it's time for O2 sensors (it has 83k on it) but first I guess it's time to check the MAF and vac lines.
Since a lean condition = too much air/not enough fuel, I guess a small vacuum leak could well cause that at idle when the engine needs the least air.
The car is stock except for an exhaust. I'll let you guys know what I find.