Once again, thanks for all the helpful responses!

Didn’t realize those bolts on the back of the cassette unscrewed. Since Sheldon said to put the spacer on first, I removed the bolts put the spacer on and then the cassette.

I took the bike for a test ride this morning and noticed some noise (a little rattling… sounds like something is loose) coming from the back wheel on bumpy terrain (I ride in an urban environment where I encounter cobblestones, bricks, concrete pavers, uneven and patched roads, manhole covers, potholes, etc). With the removal of the bolts, could the cassette be rattling slightly? Or maybe something else is loose? Mechanically I think its fine as is. Shifts perfectly and nothing feels loose… just the sound. That said, I prefer the quieter ride I enjoyed before. If the consensus is that it is the lack of bolts causing the noise, I’ll add the bolts and move the spacer to the outside or keep the spacer inside and grind grooves in the spacer for the bolts as joejack951 suggests.

I don’t think this is causing the sound but in the interest of full disclosure I also changed the chain last night. The old chain had 54 links. Using the calculation method and using the largest cog / largest chainring plus two links method I arrived at 55 links. I’d never had problems before with 54 but decided to go with 55 as I figured it’d be easier to shorten the chain than lengthen the chain if a change was needed. Could a looser chain result in more noise?