Most cartridge bearing hubs aren't as hard to work on as you might think. You can replace the bearings in Formulas (and others) with not much more than a hammer and something like a socket that matches the outer diamerter of the bearing. I seem to recall there was a really good how-to with pictures once posted in the DIY section.

Edit:

Here we go: http://www.bikeforums.net/showthread.php?t=347132

Check for a loose spoke first. If not that, take the wheel off the bike and turn the axle with your fingers. If you can feel any side to side play or crunchy resistance get some replacements (Enduro is a common and good quality brand) at your bike shop. Formulas use a common size, I think, so they should be easy to find and cheap (about 3-4 bucks each)