The fan is only a few inches from the hard drive. Fan is likely the source for the noise.
I think you cannot tell the exact location unless you open up your iMac.
But, first I would suggest that you can blow air (from a can-o-air) into the cooling slots.
Cooling slots are in the center of the back panel, immediately below the stand hinge.
Blow from as many different directions as you can, hoping that you can dislodge whatever is touching the moving fan.
If that doesn't cure your noise - your new iMac needs to be disassembled to get at the fan (or, unlikely though it might be, the hard drive.)
Your next step then is to take to an Apple store for warranty service (where it will be opened up to find out where the problem is)