Some keys on my X3 produce a ‘hissing’ sound when pressed. It seems to happen on the same keys (F and B) in every octave on the keyboard.
Any ideas anyone ?

The X3 keyboard has a rubber band below which makes contact with a PCB. It might be that the rubber is no longer so flexible and a little bit cracked, so that might be the cause.
On the contrary, I'm not sure if this is the cuase, because you say it happens the same to all F and B keys along the whole keyboard.

Anyway, keep in mind that the X3 keyboard is not very long lasting compared to other Korg's higher level keyboards (M1, T3, Trinity,...) so at some point in time you will have to clean the contacts and then later change the rubber band.

Cheers