Just recived my new 21.5 iMac with fusion drive, and I hear a whirling noise, as if a disk was loading? Very disturbing???

A Fusion drive is a tiny SSD combined with a conventional hard drive. Also called a "hybrid" drive. The conventional side makes the same noises as every other mechanical hard drive.

My iMac has a factory SSD and it is not completely silent. None are. iMac have cooling fans.

If you bought it directly from Apple you have 14 days to return it. Please replace it with a model that has a minimum of 16GB of RAM and the largest SSD you can afford though!!!

If you bought it from a reseller speak to them about an exchange.