The last few days, I've been occasionally hearing a low rumbling in the moderate distance, as of a vintage heavy bomber or two. Not having a kitten against my ear at the time, I suspect that someone's flying around in maybe a B-17 or -24 again.
Well... Saturday morning, if memory serves, I heard that sound and headed for the back yard for a look. 'Twas a brace of Ospreys, heading for Moffett.
So, yeah. Four great big airscrews, which happened to be oriented more vertically than horizontally. The sounds of WWII are new again!