If you're absolutely sure it's coming from the fork tops it can only be one of the following:

-Top nuts not tight
-Top nut 'O' ring(s) perished, nipped/unseated or missing
-Faulty sealing on preload adjuster(s)
-Bottom part of preload adjuster has fallen off (which has happened to a few 2012> bikes with the raised adjusters)

However, if the fork tube is allowing air to escape the air gap (effective air spring) will be all wrong and you will notice poor handling. If it's letting air out, but not in the forks will pack down and become unresponsive. If it's letting air out and back in then the will be no air pressure created when the forks compress so there will be no progressive stiffening - it will dive more under braking and be considerably more prone to bottoming out.

Get the front of the ground, take off the fork top nuts (mind out for spring tension) and inspect everything: 'O' rings, adjuster mechanism, circlip etc. Put them back with some silicone lube on the 'O' rings and tighten to the correct torque and see if that makes a difference.

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