Super loud modem fan nightmare!

You have a fan that is about to fail.  It shouldn't make any noise you can hear.  When the fan dies, it will overheat and won't work very well, if at all. 

The quick way to fix that is just an exchange at an Xfinity store if there is one by you.  You can also do that by mail:  https://www.xfinity.com/support/articles/returning-your-equipment 

Were you able to find the nearest Xfinity Store to you to swap the equipment at? We have a store locator here and at most locations you can schedule an appointment before your arrival.