I've had to replace many freewheels because of chain pop, which typically is heard as a knocking sound in the bb area - this occurs at regular intervals but seldom (if ever) is it precisely synchronized with crank rotation.

Your use of the word "about" leads me to believe yours is somewhat asynchronous also.

It is usually worse under load and never reproduceable on the stand.

The solution for FW chain pop is a new chain and HG FW.