What should consider for the DC fan of the frequency generator?

Now I want to use a fan for my frequency generator.
I am fraid of adding noise by the fan. The following is a picture from one fan datasheet?
What is the most important for considering the dc fan? The RPM or the sound pressure?
Thank you very much.

Brushless fans actually have AC drivers in them that might give emi pickup problems. I would trade off a DC brushless with a DC non-brushless design. If it was a low phase noise design, I would mount the fan on 4 rubber isolation mount stand-offs. If you find individual components, like VCO cans that are being modulated, you can glue a block of something on top of the can to stiffen it.