C220 sounds like a vacum at start and consistantly low on coolant
 

when ever I start my car I here this loud vacum like noise which only lasts for a couple of minutes and then is gone. It also does not happen all the time. I also have had my coolant light come on repeatedly. As far as I can tell ther is no fluid under the car. I cannot find the leak any where. I can fill the coolant up and in two weeks it will say it is low and the resivoir is almost dry. Any help with these problems would be great.
Thank you