That's just basically the snorkel for the air intake. The throttle body, where air is sucked in to mix with the gas, is in the front of the engine, driver side. A long tube goes from there to the air filter box in the back (I guess they put it back there because it was the only place with enough space). However you don't really want to be sucking up air from deep in the engine compartment so they ran that big plastic piece back up front for fresh, cool intake air. It seems to me from what you're describing that either:
1- the engine has something wrong with it, is running crappy which is causing this plastic piece to vibrate or
2 - the piece has come loose on it's own and is vibrating.
If the engine is running OK and you don't have a check engine light on, I would tend to think it's #2. The plastic piece slips into the air filter box in the back and is held down by a wingnut in the front. The wingnut may be loose or missing or the air filter box may have come adrift. Neither is serious and you should be able to drive in this condition with no problems. However loose pieces can and do end up on the highway so you might be well served to throw the plastic piece in the trunk until you can get to a mechanic or a friend or relative with some automotive mechanical aptitude. I can't imagine that a mechanic is going to charge more than $20 for this repair, heck if you're a regular, he'll probably do it for free.