Odd sound like a semi on the freeway...
 

Hey all, new here. Sometimes when I turn my car on (04 Grand Marquis Ultimate) I notice a fairly loud blatty sound that lasts for a few seconds that stops. You know the sound of a semi on the freeway as it slows down? That's kind of what it sounds like. Is that the air leveling suspension or whatever? Just want to make sure it's nothing serious or detrimental, or the cars way of saying "you just bought me and now I'm going to take a crap"

could possibly be the oil pressure building at start up... figure the engine needs to get going to pump oil into the rods and around crank after sitting...

That could be it, I've just never heard that sound before in anything else before. Sometimes it occurs even when the car has been off for only 10 minutes or so.

While I can't be certain your issue is the same, I just bought a 2004 Mercury Marquis GS last weekend which is exhibiting the same sound, usually from the right side, near the firewall.

When I got all the fluids changed, like you do with a new car, the tech said it was the AC compressor. Turning off the AC and heat (turn it to off), the problem went away, so I'm guessing he's right.

Hope it helps.

Wayne