I've got a new to me 99 Exp XLS. 4.0 4WD, 67xxx mi. Occasionally I pull into work and will hear a groaning sound, very similar to the sound of steam escaping from an over-heated radiator. All guages are fine. A slight tap of the throttle and it's gone. I'm usually getting there just in time with no time to spare.
This morning it happened and I did have some time. In stead of tapping the throttle, I popped the hood. I tracked it to the air filter box. I popped the lid on it with the engine still running and man did it ever get load. Then it just quit making the noise, just as I got the box fully open.
Could it be doing that because it was trying to suck air past a dirty filter? There are a lot of things hooked to the breather tube that connects the box to the throttle body that I don't recognize(I'm Old School). Is it possible one of them do-dades can make that kind of sound?
I honestly don't know what's been done when on this truck, so I'm gonna replace the air filter for good measure.

Anyone heard of this before ?