Another stupid question.

I bought a new cassette for the new wheels, and the smaller 4 or so cogs are rumbling. I figured I needed a new chain, so after ordering one, decided to put the old cassette on the new wheels. Then I noticed it has 5 spacers instead of 3 on the new one. I had doubled confirmed that everything was in the correct order.

Old cassette is 105 (R6800, I think) 11-32

New cassette is Ultegra (R-8000) 11-32

I’ve got UDI2 shifters, couple years old, not sure of the number at the moment.

Alright, so do I need more spacers for the new cassette, or what’s the story? My attempts at adjusting the UDI2 shifting did not improve anything (with the new cassette). Any advice would be appreciated, especially if it’s the correct advice.