Ok...I'll add my 2 cents worth. I completely eliminated the noise coming from my shimano freewheel by taking my angle grinder and knocking some length off each tooth, then grinding a bevel onto both sides of each tooth. What was happening was my chainline wasn't perfect...(and how many of you have absolutely PERFECT chain lines?...be honest!!!) and since the teeth of the freewheel protrude significantly thru the chain links...and have absolutely no bevel, the freewheel tooth tips were ever-so-slightly catching the chain side plates as the chain came over the top of the freewheel. If you inspect a road cassette, you'll notice the cog teeth are significantly shorter than those on your freewheel and noticeably beveled on each side to accommodate not only the shifting but the chain angles that result from shifting the rear. So...basically, I took my angle grinder and made my freewheel teeth look just like the teeth on my 8 speed road cassette....much shorter (they don't protrude all the way thru the chain anymore)...and beveled on each side. The shorter tooth and the angles on each side allow the teeth to completely clear the chain side plates as the chain comes over the top. I even did this on the cog for the fixed side...and now there's absolutely no noise what-so-ever on either side.

I've been running both my bikes...a fixie/SingleSpeed...and a 3 spd conversion road bike(ultegra triple 52/42/30 on the front...and a 17T freewheel on the back) for a year now with these ground down freewheels and have had no more noise...and absolutely no problems in over 3,000 miles combined from both bikes.

A quick way to tell if your chain is catching the freewheel teeth, is to look at the sides of the freewheel teeth...if they're polished (typically on one side), that's caused by the chain plates dragging across the tooth tips when they come over the top of the freewheel...and if the teeth are wearing...then the chain is as well...

Hope that helps...