Originally Posted by riderx
Simple answer: Cheap freewheels. Unless you use a White Industries you can expect this to happen.
Indeed, only the most expensive parts can possibly work at all. Also recommend replacing your cranks with Sugino 75's and getting a Thompson seat post and Toshi straps just to be safe.
Two things can rattle in a freewheel, the pawls and the cog. The pawls will sound like something very small (because they are), but they've got a spring holding them down so they won't rattle unless you shake it really hard or hit a bump or something. This is normal unless it sounds like there's something rolling around in there totally loose, if it bugs you you can take apart the freewheel and use heavier oil in there (phil tenacious or similar), but there's no performance gain to be had and disassembling freewheels kind of sucks. The other possibility is that the cog is loose because the cone assembly isn't tightened down enough, which would be indicated by lateral play. You would need to use a pin spanner (or a bent coat hanger) to tighten it. Sheldon explains it here: http://www.sheldonbrown.com/freewheels.html#disassembly
He says to hit it with a hammer and drift pin, this may be necessary but if it has actually loosened you can probably do it with a spanner. He explains how to take out spacers as well, this is always a possibility but shouldn't be needed.