A HDD is responsible for 2-4 watts idle and up to around 6 if you are pounding it with reads/writes - it's not a huge difference when idle power is ~30 watts.
I never recommend downgrading fan specs / putting a noctua in a vintage machine. As
@joshc said... these aren't intended for silent running, and the classics in particular with their combined analog & power supply board were never the most reliable to begin with. Cutting corners on cooling is asking for trouble.
If you want to make things quieter, you likely could design an adapter to use a quieter 80mm fan in place of the 60mm while moving the same or even more air. Folks have done this for the Color classics already, iirc.
For SE/30, especially a fully upgraded one I recommend this fan:
Pico Ace 25 60mm Brushless Cooling Fan - 12V DC - 15.5 CFM - 25dB 109R0612F415 . Cheap to find NOS, made in japan, and it's the upgraded version to the sanyo model originally shipped with the machine. Power consumption and sound levels are about the same, but moves about 20% more air in my pseudoscientific tests. Upgrading to a higher flow fan is about all you can do to improve airflow unless you want to cut holes in your bucket.
Biggest thing you can do to help cooling on a SE/30 is a Seasonic PSU - the efficiency improvements help
*a lot*. On the same upgraded machine, I found 10-12 watts less drawn at the wall changing to the seasonic. Here's some data I have:
SE/30 #1: 128MB, 2GB HDD, Micron Grayscale, Maccon, Diimo 030, original Sony PSU: 60 watts @ finder
SE/30 #2: 32MB, 500MB HDD, Interware Grandvimage, Maccon, Carrera 040, Seasonic PSU:
55 watts @ finder
Every piece of #2 is more power hungry but it stills ends up consuming less power - that efficiency goes a long ways. Unfortunately, this is not possible on the classics without serious hackery.