It may indeed be the case that you are 100% right and I'm 100% wrong about the root cause for the noise increase (I'll freely hand it to you - the odds are in your favor!). I would still be interested in seeing the difference between different fans - seeing as how they all have different free-air noise characteristics and so forth! Can fan A go from 12db to 16db in the same situation that fan B goes from 12db to 18db? Yes? No? Speculation: A fan with a pronounced hump in the spectrum could behave differently from a fan with a flatter spectrum?
What you are really talking about is static pressure performance. As fans face static pressure, their performance decreases and their sound increases.
Generally the most static pressure PC fans face is when they are used for force air through a heat sink. If they sound quiet there, they are going to sound quiet in most other PC applications. As a practical matter, that it probably a good test.
Whatever is causing your noise, it isn't back pressure or static pressure.... at least not anything approaching the static back presssure one would get from the CPU heat sinks that SPRC uses for its testing purposes.
If the cause is resonance... any test for resonance will be a one off test and be useless for the purposes of reference to any other case or case configuration. Only you can test for resonance in your own case.
I hope that makes sense to you.
To the extent that you have a backpressure issue, go get a Scythe Gentle Typhoon. It doesn't generate than many CFM in free air, but it just keeps truckin even against backpressure that would bring otherwise higher performing fans luffing to their knees.