Daikin AC clicking every 2 hours, high pitched whine, L Series air conditioner
There is a relay in the indoor unit that normally energises when the unit is started up. However when idle and in 'standby' mode, this same relay energizes briefly approximately every 2 hours, for no apparent reason. It is very noticeable when the unit is installed in a quiet area such as a bedroom.
Models in the L-series (e.g. FTXS20LVMA) are affected.
A Daikin technician has confirmed this behaviour is normal. They offered to modify the control board by cutting a circuit link to stop the clicking, however they claim this will increase standby power consumption.
It is quite a strange behaviour. Presumably the engineers designed it so the unit 'wakes up' every two hours to do a self-test or analyse environmental conditions. But why use a noisy mechanical relay? Using an opto-isolator and MOSFET / Triac instead of a mechanical relay would make it silent. Maybe it costs too much? There shouldn't be an excuse for a premium brand of arconditioner.
Another issue is when not in operation, the indoor unit emits a faint high-pitched whine. It seems the switchmode power supply in the indoor unit is poorly designed and the transformer emits the noise when the power supply is lightly loaded.
See also: Daikin Web Remote Control - WiFi & Arduino