I have an unusual noise from my split system air con indoor unit.
Let's say I have the unit in cooling mode, but it has reached thermostat temperature and so the coolant has stopped pumping from the outside unit (I'm assuming this is how it works). When the coolant pump kicks back in again, a noise starts up after about 1-3 minutes of operation. The noise is coming from the evaporator coil in the internal unit. I can replicate the noise by running my fingernail along the cooling coil when the unit is offline. It is a metallic pinging noise, with the same frequency (occurance rate, not sound frequency) as if you ran your thumb over the teeth of a hair comb. It lasts for up to a minute and then stops.
The problem is that it is just loud enough to wake us up at night and results in bad sleep from an otherwise good air conditioner.
Does anyone have any ideas?