without being too technical,i am no mechanic the shim sits on the end of the valve & under the valve lifter on the camshaft end.
when the valve clearance has exceeded range then they need replacing....

As you need to measure the gap then order shims which then maybe the wrong size best left to the pro,s who should have various sizes in stock.
Yes this will be a light rattle especially when cold coming from the top end
This is what i am led to believe i may stand corrected