[MT-AU Public] Spinning disk like noise on RB951ui-2HnD

Hi, I've got one of these little beauties (RB951ui-2HD) at my home for my QOS and L3 load balancing. I'm curious, only recently, when any traffic goes through the device at over 10Mbps there's a noise which sounds a lot like a spinning hard drive under heavy load. Has anyone noticed this/is this normal? I'm not overly concerned, just a little curious. Right now it's running PCC with failover like a champ, I'd only wish I could do PCC and PCQ.. CPU is not even going past 30% Cheers, Andrew.