Clunking noise only when pedaling at higher RPM not low RPM. Video of Issue linked

Given that the cassette lock ring was torqued properly and the play between the cassette and free hub was similar to that found on friends bikes, I moved on to checking other parts. I removed, cleaned and lubed both rear derailleur pulleys. This reduced the clunking sound about 90%. The pulleys currently installed are original and I'd guess have about 700 miles on them. In any event, I plan on replacing both pulleys with new ones with the hope that this completely eliminates the clunking sound.

My guess is the old pulleys weren't turning as freely as they should and this resulted in less tension on the chain, which created the slack between the cassette and hub that enabled the clunking sound. My thinking is the new pulleys will have a slightly larger diameter that will allow the rear derailleur spring to provide just a tad more tension on the chain thus completely eliminating the clunk. The reduction in chain tension may have resulted from a combination of pulley wear and slight chain stretch (but not enough to register on a chain stretch tool). New derailleur pulleys will do the trick. Thanks for taking the time to read this.