If you will look at your front axles, they have rubber boots on them. Two for each side, and inner & outter. These boots are full of grease for the CV joints they house. When a boot goes bad, they will sling all the grease out of them, and make a noise just like what you are describing. This doesn't mean your CV joints are bad (they could be) but it does mean they need looked at right away, and a new boot & grease installed before they do go bad.