i had similar issues , but never figured out how to solve them, i even switched from t-mobile to orange and to 3 g also because of higher connection speeds , these managers all drained processor power between 15 and 30% , thats just how usb works , there are some bits transfered via usb from the dongle , its like if you copy a file from a external harddrive , it simply will use processing power , and as faster the connection the more bits are transfered , the more processing power is used thats usb, but most people just do not care and dont even look if its using processing power
after 3 month getting annoyed about that i got a landline and wireless router and ethernet connection and problem solved
but for short periods even ethernet will use processor power if you open a website , thats how the computer works everything you see on your screen and every app open in the background will use processing power to some extend , thats unavoidable
and the spinning fan hmm mine is only spinning fast when its getting to hot ,at 100%usage, but below 100% its quietly spinning a bit i can only hear the fan usually if i press the ear on the case as i rarely push the mini to 100%