Why does the natd process hog my CPU? When sharing internet in my Macbook, from a 3G USB pendrive to Wi-Fi, if the 3G connection falls, the natd process rises up in CPU usage.

Is this a real problem? Is it expected in some way?

There's a easy way to solve the hogging, using the menubar disconnect option, but sometimes it doesn't work and I have to unplug the USB modem.

My modem is a HUAWEI E226.

Hope you can help me.
natd should be runnng ... NAT means Network Address Translation, so it's basically translating (communicating) between the Modem and the Wifi Access Point to share your connection.

You can read more about it here, natd 

So in short this needs to run to share your connection,

Hope this helps.

Karl