Strange Noise From Air Intake

Getting a strange gurgle sound in my stock air intake box and air tube..I've got a stock 04 GT 4.6 L with just a K&N air filter.40K on this very well taken care of machine.No unusual codes.idles fine,performs well no misses . The sound is intermittent and comes and goes while idling. Have cleaned the mass air sensor with the correct spray with no change.Tapped on the idle control valve with no affect.This is driving me crazy...car still runs great but i'am concerned that this may lead to something worse . Sensors are too expensive to start "parts changing arbitrarily ...any ideas will be appreciated .Thanks

Update...changed PVC valve OEM ev261..and new set of Motorcraft plugs sp493...no change...sound comes and goes at idle..no actual change in idle itself and the engine still performs flawlessly !??????????/ my granddaughter who heard the sound when she was standing next to the car said it sounds like a little motorboat Pop Pop .Now i notice what sounds like a miss from the exhaust on the passenger side . Any ideas ....once again no codes on the diagnostic machine ??!! HELP