Lol yeah, it's a bit of dust.

These Corsair fans at anywhere near 100% are really quite loud, whereas I can just leave these Sythes at 100% and never know they were on and get their full performance.

Corsair Fans
Fan speed: 2700 RPM
Fan airflow: 77 CFM
Fan dBA: 37.68 dBA

Sythe Ultra Quiet
Fan Speed: 1600RPM
Fan airflow: 68CFM
Fan dBA: 28 dBA

I'll post back when I get round to changing them.