Cubase Pro 9 speeds fan up on iMac

Hi, I am using Cubase Pro 9 and a new iMac, 4Ghz, 32Gb, 3Tb HD.
When using cubase pro 9 for mixing I am finding that the fan on the iMac speeds up quite a bit and is very noisy.
This only happens when using cubase.
I’ve checked my CPU and I only use around 40% maximum.
Has anyone else had this issue and have you been able to sort it out?