And it has a steady and distinct pitch, too. I hum it, walk over to the piano. Could it be an A? Always a good first try; a million tuning orchestras can't be wrong.
But no, it's not an A, or a C or an F. Better try some black keys. And there it is — a B flat — or at least my out-of-tune piano's version of that pitch.
Were I of a more mechanical bent I would worry about what's making this sound. I would check for leaks or breaks. But instead, I listen. I let the rain sing its song.
(Waiting for Sächsische Staatskapelle Dresden to arrive and tune in the large golden concert hall of Vienna's Musikverein)