Ford Truck Enthusiasts Forums - 2008 Expedition Noise

New 2008 Ford Expedition 5.4 liter engine - Purchased Thursday, 8/28/2008
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Concern: After the auto has sit for a somewhat short period of time and/or overnight, I will hear a rather loud noise which sounds similar to a vacuum actuated plenum door slamming shut when accelerating somewhat aggressively. The sound will not be heard again, no matter how aggressive the acceleration, until the auto has sit idle once again for a short period of time and/or overnight.
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Note: If the auto is driven cautiously (using the "light foot" method), the unusual sound is NEVER encountered. The sound seems to be coming from either just behind the driver's instrument cluster down near the floorboard OR just underneath and/or in front of the driver's feet on the outside of the car (possibly near the catalytic converter or brake booster pump reservoir). What can this be ............. the dealership doesn't seem to have a clue at this point in time AND I would rather they not simply start changing parts in an effort to resolve the issue.

UPDATE UPDARE UPDATE

I think I might have determined the cause of the problem ......... I feel the the sound is coming from the power brake booster.

I have been purposely letting my foot rest ever so slightly on the brake pedal when I first take off in the morning. Every time I hear the unusual sound, the brake pedal seems to "pulsate" slightly at the exact same moment. Since the brake booster contains a rubber diaphragm inside, I am thinking this is probably the culprit.

Anyone else ever have this type problem in the past?

Is a power brake booster easily changed?