Gurgle at start up?

I have a 2000 A4 1.8Turbo Quattro that I bought second hand. At start up, there's a gurgling noise that seems to come from the rear end. Any thoughts on what this might be?

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I get it too. This link explains.
http://www.audiworld.com/tech/misc8.shtml
Richard
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Huh...wouldn't have thought about an upside-down U-shaped gas tank. Thanks for the link!
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