And, is it not just bearing noise -- needs at least a cleaning, preferably replacement?
I don't recall those scopes being particularly noisy, though it's been a while since I used one.
Better way to work with the fan, probably: take it out and power with a bench supply or whatever. Hold it against the table, does it vibrate significantly (table as a sound board)? Does it change if you press on the rotor (watch your fingers!!), or move it around or whatever? Is it buzzing at the rotational rate, or a harmonic (blade noise), or more generalized (white noise, the rushing of turbulent air)?
Replacements are easy, anyways; even if it's not a standard size, it's probably awfully close to one. Check drawings for bolt holes. Probably safe to use somewhat less CFM, or higher (say 30+?), I don't know that those things got all that hot inside? Otherwise, equal or greater to be sure about it. And just being ventilation, output pressure shouldn't be a concern. Can opt for ball bearings (or air or magnetic as the case may be?) for long life, and datasheets/supplier catalogs usually list acoustic noise even, so you can sort by that. Blade noise depends on shape, so there's some room to control that while keeping CFM up, but generally speaking higher CFM will of course be noisier, so you can't be too picky at some point.
Tim