I hear it when I really push to empty my lungs, when I’m in the pool, my ears are under water, and sound is magnified. I hear it “internally” like you might hear your heart pounding in you ears (but it’s not localized in my ears, but in my chest). I don’t hear it when my ears are in open air.
I’m getting conflicting sensory information. It doesn’t feel like it’s fluid in the bottom of my lungs, it doesn’t seem to interfere with my breathing, my lungs don’t feel congested like when I’ve had a bad cough or cold. Yet it is clearly and closely linked to my respiration, begins when I am near the bottom of emptying my lungs, and ends as I begin to inhale. I somehow imagine that it is a small pool of fluid being pushed from one little space to another by the muscular contractions of exhaling, and seeping back to the first space while on the inhale cycle. Kind of like a slow knuckle crack.
A little health history if it would help – I’m 43, about ninety days smoke-free now, I have high blood pressure and a hiatal(sp?) hernia. I have been more physically active this summer than I have been in years. About 3 times a week I do a 6 mile round trip bike ride commute to work and back. I swim a dozen laps at the pool (admittedly not a large one, I don’t know its length) about five days a week. I do fifty situps every other day. Not a much in absolute terms, but practically Olympian compared to my level of physical activity in the past 5-10 years.