Normal D5 Pump is silent up to 70%. Than some noise is normal.
There is a good chance that D5 with Speed Control without new changes etc could work on speed 4 in closed case without noise.
Even if you hear some nice many times D5 after 24-48h use on same speed is less noisy then when you start first time.
This is pump you should looking for. If you find someone reliable worth paying similar price as brand new EK or XSPC, etc..
Only little less because you will not get warranty 1 year. But they are reliable.
Even if they are not maybe from original factory they don't have changes latter applies with manufacturers who bought them to sell to computer enthusiasts.
These are numbers of Laing D5.
You need D5-38/810N that's not strong version.
As you see on picture Part No is 71629