I go mad, I go mad, I shall wear my trousers plaid.
After a day at the table, I have a new theory on the low-frequency noise. A fragment of the acoustic spectrum of the trams, but without the vibrations, with the apartment being sufficiently above the street so that it's not recognizable, and perhaps on some sort of a harmonic with the intersecting rail lines. It doesn't explain why I only noticed it in the last few days, but it seems the most likely explanation. Or, you know, it could be the invading aliens here for our 1950s rock and moonpies.
Onward.