Well the static air pressure can also have a big effect on the noise of a fan too. In my homebuilt x86 NAS project i was trying to cool it all by drawing air trough the small case using a 120mm fan, but no matter what fan i used i could never get it to run near silent while also pushing the required amount of air trough it. Eventually i ended up grabbing a dremmel and cutting out the hole pattern where the fan mounts and blows trough so it was now a wide open hole, this finally solved it and got the noise level down to where i wanted it, now the harddrives are louder than the fan when all of them spin up.
So the restrictive air path could also be one of the things that is making the fan loud, or making turbulent noise as air makes its way trough the restrictions.
Oh and looking at the datasheet for the HP 3577A it claims it uses 450VA max, so that's indeed quite a power hungry instrument, ofrcouse the real power use in watts is likley quite a bit lower, but i think its going to be quite difficult to cool without making a lot of noise. The big ass HP 8566B Spectrum analyzer i have has a power consumption of 650 VA, sounds like a hair drier with the powerful fans and yet still gets pretty hot.