High pitched buzzing coming from unit

My original Ten64 unit has developed a high pitched buzzing noise. To the best of my knowledge, it seems to be coming from inside the unit, rather than the power brick. I will swap the power brick over today when I can have a bit of downtime, but does anyone know what would be causing this?

It’s the fan at approximately 5500 RPM (?) although it’s not like any noise I’ve heard from a fan before. Nothing caught in the blades - @mcbridematt what’s the part number for the small Noctua? I’ll order a replacement ASAP, it doesn’t look like I can keep the fan under 4000RPM

It’s the NF-A4x20 PWM (12V, 4-wire) model.

If anyone in Australia needs a replacement, we have quite a few of them sitting in our office.