Cabin pumping noise

I have 2002 4L Ranger.  I have recently noticed an odd noise in the cabin.  It is very soft and only easily heard at an idle.  It is present while driving but tougher to hear due to the engine.  It reminds me of a leaky air pump.  It is very regular, and has a "two beat" cycle to it that never varies.  Does anyone have any ideas as to what may be casuing this noise?

Thanks in advance.