Well, that worked great. A nice consistent/smooth fan sound. It's quieter, more efficient, and the buzzing is gone.
My fan holes were 3mm.
I had to severely cut up the rubber grommets from Noctua to fit.
I wasn't down to drill out the holes in the instrument.
Alternatively I could have used some 3mm BHCS to mount them (original screws are far too short)
But I seeing Josh' picture I decided to give the grommets the opportunity.
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If I take pure fan/air noise, the Noctua is louder than the original fan IMO.
The original fan had an annoying buzzing which made it far more annoying, but if you ignore that, the pure fan noise "part" of the sound is either same or slightly higher on the Noctua.
Overall I still think the upgrade is worth it, the fan moves a LOT more air and you don't have this annoying buzzing noise constantly going on.
Also Noctua fans are rated for a LOT of hours, it is unlikely the instrument will need a new fan in the next 20 years based on how much I use it.
Noctua is really up there in terms of their fan engineering (quite a few very nice videos about it on YouTube).