Hi Bunwich,
Depending on the design of the panel buttons, an audible click is completely normal, e.g. also with Genos and Tyros models. Some PSR models have the well-known rubber buttons, which are quiet but feel a bit "spongy" (imprecise). Fortunately, Yamaha has returned to buttons with a clear, defined pressure point (and thus an audible click) on the PSR-SX models.
Best regards,
Chris