Open-cell foam, the kind used in upholstery and tool cases, works surprisingly well as a fan holder and vibration insulator. It's my preferred solution.
The one downside is that using hot wire foam cutter releases noxious fumes, and if using scalpels, the foam dulls the blades faster than anything else I've cut. Very fiddly work, too.
(I've thought about suggesting the foam block solution for those wanting a really quiet cooling solution for their Rigol DHO800/900 series oscilloscopes, mounted outside using the VESA100 mounts. You do need to drill new big holes in the fans for the VESA studs, and "isolate" those studs from the fan with foam cylinders, if you also want to use the VESA100 mounts to hold the oscilloscope in e.g. a monitor stand. Choosing a 3-pin fan that is silent but has good pressure gradient capability at 8V, should make it an excellent solution.)
When fans produce noise without distinct peaks in the frequency spectrum, the "beat" issue cannot happen. The issue is easy to hear: instead of a "constant" sound, you hear a "beat", when the distinct frequencies generated are off by less than a few Hertz. Its mostly a problem in push-pull configuration, between the pushing and the pulling fan across a heatsink or similar barrier, and especially when they are driven from the same PWM. Then, the small difference in airflow causes the pulling fan to rotate slightly slower, causing the "beat".
When fans are in parallel, the combined airflow tends to "sync" them, so that when driven from the same PWM, they tend to have exactly the same RPM. Unless, of course, one of them is damaged/older/et cetera, or has an a small blockage to its airflow compared to the other(s), and for that reason runs just slightly slower.
Noctua nowadays sells pairs of 140mm fans (
NF-A14x25 G2 PWM Sx2-PP,
NF-A14x25r G2 PWM Sx2-PP), that are offset by about 25 RPM (≃ 0.4 Hz) at the same PWM duty cycle, exactly to avoid the "beat" when driven from the same PWM. Although Noctua says it can happen when fans are in parallel, I personally have only seen (heard!) it in push-pull configuration through a heatsink.
I learned about the workaround when watching machining videos, back when Noctua's largest PC case fans were still 120mm. CNC mills and lathes vary the RPM sinusoidally around the target to reduce chatter, which is a resonance phenomenon very similar to fan "beat".