It sounds to me like the pump has given up. The water still fills in the bottom tray but is not being pumped and trickled over the pads.
The release valve is still working. This is the tray water you hear bring dumped when the system is turned off.
Zero is right. You need to take all the pads off and see it it's 'raining' when the unit is turned on.
That's why I hate evaporatives. One has to get up on the roof. Age may not allow for this. Plus they don't work with high humidity.
If the house isn't massive...get a couple of refrigerative units. Or bite the bullet and get ducted reverse cycle. The current ducting has to be replaced as it's too thin. I realise refrigerative is more expensive to run and it needs the place to be locked whereas evaporative allows windows and doors to be open...handy for those with kids! And handy to allow nature's breeze in.
ALSO...be careful of rough tradies on your roof...especially if tiles.
*** Not having a go at you troopy...you sound decent ;)
