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Bad gas and slight water condensation in the tank maybe after sitting ?
The exhaust flap, as your neighbor called it may be located on the passenger side exhaust manifold. It consists of a cast iron block with a movable flap/valve inside, controlled by a rod and vacuum cylinder motor. It pulls the valve (flap) closed when the engine is cold. The idea is/was to force more exhaust though your intake manifold to heat it up quicker. Then it opens up when it vacuum is cut by a port on your emissions system. The default position should be OPEN. If it closes during regular operation you could hear some screaming... Test it by taking a look at the position of the control rod right after you start her up COLD, and then again after she is at OT. That's assuming you have one at all!