I suspect that with a standard vacuum motor, a lot of the noise comes direct from the electric motor itself, as well as the fan. Electric drills and other similar power tools with high Rpm ac/dc brush type motors, all kick up a similar racket.
An ac induction motor as fitted to a large single blower will run virtually silent. There will always be some air roar, but venting the bench outside, or down a stormwater drain (as I have done) is well worth the trouble.
Lining your motor compartment with foam rubber may help. If the panels vibrate, some sheet lead may make a difference, but I have never tried this myself.
If your test piece is noisy, try to discover why. If it flows efficiently it should not whistle.