About fans and fan noise in your Apple product – Apple Support (AU)
Find out how your Apple product monitors internal temperatures and uses fans to cool critical components.
support.apple.com
If your device's processor is working on intensive tasks — such as compressing HD video, playing a graphics-heavy game or indexing the hard drive with Spotlight after you migrate data — the fans run faster to provide additional airflow. You might hear fan noise when this happens, especially if you're in a quiet environment. This rushing-air sound is a normal part of the cooling process.
If the machine was indexing during GB5, it was probably still indexing during Cinebench R15. Indexing a new machine can take hours. Especially if you are constantly loading up the CPU at the same time.
Just sayin'