John,
I am just going to throw out a few ideas. First, is the sound like 1) metallic popping/crackling
2) like grit
3) greasy gunk that has gotten wet

Have you ever cleaned this freewheel? If you want to try to continue using it you might clean it by soaking it in solvent for an extended period, swishing and turning it periodically. I have seen written that this should never be necessary, but other sources describe this process. After thorough drying you dribble a few drops of thin oil throught the juncture between the fixed and turning part.
Assuming the freewheel has some miles on it and has standard threads, I, personally would just buy an newer SRAM or Shimano freewheel. Harriscyclery.com has a nice selection of both. Nashbar and Performance have SRAM. If you don't have the appropriate removers, just get the freewheel from a shop, and they will spin it on for you. The shop can also look at it and make sure they get you the right thing. New freewheels are not that expensive - $20-50, depending on what you want. Replace the chain with a good SRAM chain at the same time.