If it's a Shimano freehub then it's possible to take it off and dissassemble it, clean the insides out, put some very light grease on the bearings and some light oil on the pawls and then assemble it back together. Most people don't do this and it's not reccomended by Shimano.. but it can be done. The easiest thing to do is to buy a new freehub body, they're not very expensive.