Your mate's ear was only a few Hertz out if he thought it was a D- on 440Hz equal temperament that would be 294Hz, whereas the fridge hum harmonic (in this case good for both European 50Hz- 6th, and American 60Hz- 5th) would be 300Hz. I'm a bit surprised he didn't hear a B (62/123.5/185/247Hz).

I wonder why buses were in c#? The turbo on the engine perhaps? It all reminds me of a vaguely- remembered story of some music professor at an English university. The students would torment him by playing a hanging chord on a piano in a room below his, late at night, then leaving the room. The professor would have to get out of bed, dress, go down to the room and resolve the chord before he could sleep.