rattle under the hood

hi everyone I have a 1996 328i sedan and there is this air filter type thing located on the passenger side closer to the windshield than the front of the car and it has an air hose coming off of it thats about 18 to 24 inches long and when its cold out and i turn the car on it starts rattling and making this wierd noise for the first 2 or three minutes then stops what could this be ??????