A huge thank you to everyone who posted on this thread. Special thanks to Fairview for "hanging in there with me". After your last post everything became clear and I understood it much better. I went back into the attic, took the same panel off, looked at the very same evaporator coil as the night before only this time it was obvious that the bottom of that unit is a tilted tray where any water would flow towards the pipes. I poured approx 6 cups of water into the tray and, excitedly, rushed down the ladder, put the heat on and ran to the bathroom sink. Result - no more annoying noise. I feel so stupid that the entire fix was a small amount of water poured onto a tray, something that, I am sure an HVAC Engineer would have charged me a couple of hundred bucks to do.
Like I say, big thanks to everyone and especially Fairview. Since Tuesday I have learned that I have 2 furnaces (1 in the basement and 1 in the attic) and that the outside AC units are just the condensers. I have also learned that each furnace has a condense drain system that runs to the lavatory/sink drain and that, in the case this drain gets blocked, there's an overflow system that will discharge the water to a visible place alerting me that the regular drain is blocked and this system is saving my ceilings and walls from being soaked with drain water.
Fairview, if you're ever in LA, I owe you a beer or 10 and my wife says you've restored her faith in humanity. Thank you everyone.
PS. Fairview, the reason I didn't put the system on cool 2 nights ago is because I'd already put my 2yr old to bed and he would've been freezing with the AC running.
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