Is it possible to make the BitAxe Supra completely silent?

Yes, it is. By replacing the stock fan with a Noctua NF-A4x20, you move from almost silent to completely silent without sacrificing cooling performance.

Stock: 40db, 4400 rpm, 57.0°C

Noctua: 35db, 2491 rpm, 64.0 °C

Note that this upgrade is a nice to have. At 40db, the BitAxe is already nearly inaudible. If you tuck it away in a closed space, you won't hear it.

However, if you place it on your desk, you will notice the noise. Additionally, the stock fan produces a 'pulsating' sound, which I find off-putting.