"Or its normal to hear a low noise from the fan ? Both my new 21,5 iMacs are hearable when its very quiet around... Both have their fans running in 2100RPM, using fancontrol.
So , when its very quiet around, can you guys hear something ?"
Of course it's "normal".
If a device has moving parts, it's going to generate some sound.
Some computers generate more sound than others.
The iMac is "on the quiet side", but the only time it's going to be "silent" is when it's sleeping, with the hard drive spun down and the fans stopped.
I snicker when I see complaints here about "noisy iMacs". I have a g4/MDD "wind tunnel" Mac. You ought to hear that!