I have serviced lots of freewheels, it is quite easy once you get over your apprehensions.

Last weekend I was riding one of my Paramounts when I heard a loud clunking from the rear. I had heard the same noise before but it went away quickly. I got back to my starting point while it was still making noise, and once I got home and unloaded the bike from the truck I tried to duplicate the noise when the bike was on the repair stand. No noise, of course. I removed the rear wheel and found the problem - the bearing race/cover had come loose, but not loose enough to let the bearings fall out. I tightened it with a spanner wrench and all was well again.

My first road bike since the 70's, a Giant that was given to my by a neighbor, had an annoying clunk coming from the freewheel. It was actually a cog that was slipping and making a noise with each revolution. The thing was so worn that I just replaced it with another, but that could be the source of the noise.