What is this and what does it do?
 

Part #000 140 04 31
1996 C280

It makes a "flapping" noise upon cold startup and can last for up to 20 seconds. I heard it is some kind of resonator for air flow. What's the story on this part, i.e., performance effects, replacement recommended, etc?

Intake manifold variable flap...changes runner length to increase HP above 3200 RPM. Shouldn't ever make noise.

M.B. Doc,

Do you mean that flapping noise shouldn't happen for this part and I should look elsewhere for the flapping noise or that it is defective and therefore needs to replaced? Sorry, just trying to clarify. You do mention that it is a "flap" of some sort. In your experience is this a common repair item or usually bulletproof? If it is malfunctioning, it'll affect the performance (high and low end)? Thanks.