I went to the bike shop a month ago and had a freewheel installed on my flip/flop hub for up/downhill purposes only. It would make a loud knocking noise at higher torques at higher speeds so I asked the LBS guy what the problem was. He told me it needed to be broken in. I knew that was a bs answer but he seemed really busy and told me he'd order another one if I still had the same problem the next day. I went back the next week (on my day off) and told him I still had the problem. He orders a Shimano cog and I get it installed the following week. Knocking is gone.
The first cog (generic brand) made a rattling noise when I flipped the hub onto the fixed side. The same with the shimano cog. It happens at all but really slow speeds.
Is it not tightened down? Is it not greased enough? Is it just a bad batch of cogs?