Nice thread this one! I just moved into a new house with a parking garage underneath our house where we park both our Model S (85 from 2013 and 100D from 2018).
My workbench/garage is in the same space and I also started noticing the S85 humming while sitting in the garage idle. This would indeed go on for a couple of hours.
Both cars charge to 90% as a default setting. I also update firmware whenever one is available, so I'm always on the latest one.
I'll keep an eye out, but I'm almost certain mine is suffering from everything else reported here.
Thanks. We now now if you charge to above and around 78% (especially for your older model S) your cooling pumps kicks in. This is a known behavior as we know it now, but was rather mysterious when I started this thread more than a year ago. Charging the older cars exhibiting this behavior to a higher SoC is a waste of electricity since the car gets rid of that extra charge. We have named it Draingate to go along with the other two _gates, namely Batterygate and the Chargegate that older model S cars, specifically S85 cars suffer from these days.