$14 for a crank puller, $15 for a multihook, few cents for grease. Rip it apart, clean it out and have a look, find out how it works and reinstall it perfectly yourself. Extra tips available at sheldonbrown.com. After doing it once, you're nearly an expert!
If the grinding is ONLY on backpedalling (i.e.not when forward pedalling) - don't panic - it MAY be just the freewheel. My own freewheel has a distinct cyclic grind when I pedal backwards but as this bearing is never under load it hardly matters. Rolling downhill doesn't put too much stress on it!
I guess the best way to check is to flip your bike over and drop off the chain and have a good feel of the movement. DO look at Sheldon Brown's website for info, even if you then decide to take it to a bike shop. I found all my bearings were WAY too tight and very sparsely greased. Makes for harder work and early failure! If you DIY you won't go near a bike shop again (except to buy tools!) If you don't DIY then get the best service you can in the remaining time...