macbook pro retina 15 fan noise

I am wondering if someone can help me with this...

Lately my macbook retina 15, mid 2012 right sided fan has started making a noise when running at idle. The left one runs silently while the right one makes a constant noise which is audible in a quiet room/environment.

I know it is the right sided fan that's making the noise when running on idle at around 2000rpm.

I took it to a genius who obviously could not hear it in the loud store... in all fairness he took it to the "back" but still could not hear it. He would not let me compare two MacBooks or let me go back with him to hear the noise.

I told him that it is functioning just fine and but they ran their their own hardware diagnostics to confirm that everything is working just fine.

so... if it is making a noise while working, then "hardware is not working as expected"... how do I get them to replace the fan because it is making noise when idle...?

Any help would be appreciated.

Thank you