Long time fix gear rider here. First, you say chain "tension". The chain should never be tight. I look for around a 1/2" of play in the chain midway. On the velodrome, with no bumps to worry about, they often run more. With less than velodrome worthy fix gear specific cranksets and cog, you have to spin the pedals enough to see both the tightest point of the chain rotation and loosest and find a compromise hub location that keeps the chain adequately tight at the loosest and never goes actually "tight" at the tightest.

Next - do your chain and freewheel have a lot miles? If so, they are stretched and the chain may not be a good fit now over the much newer fix gear cog. Measure your chain length. 6 pairs of links on a new chain are 12" exactly. Measure with a steel tape measure from front of pin to front of pin with the chain on the bike. If it measured 12 1/16" it has enough wear to be noisy but still work. If the cog is new, a new chain will run far better (but may have issues running over the freewheel).

And - cog shape. There is quite a difference between cog teeth from one brand to the next. Some get more finishing after the basic teeth are cut; others are left square. The more finished ones can be a lot quieter. It can also be a lot easier for a chain to jump off those more finished cogs. I ride the black, very plain and square Eur-Asian cogs because the are so resistant to the chain coming off. (Many on the velodrome ride the same cog, probably for the same reason.)

So: get your chain "tension" right, see if the chain matches the cog for wear and look at your cog's teeth for shape. Cogs will wear in relatively quickly to better fit with the chain and quieter ride if a somewhat stretched chain is the issue. Be aware though, you are rapidly going through the life of the cog until you have "aged" to to match the chain. Cheaper in the long run would be to get a new chain. (In a few miles, it will stretch and work better over the freewheel if it doesn't new.)

Ben