Thanks for all the input, I'm relieved nobody said "oh yeah, that's a leak." This is our first winter in the house so I'll be interested to see how the temp and humidity changes affect the sound. I don't recall hearing anything last summer but can't really say I was listening either.

DC isn't nearly as transient as the reputation. Students, yes, of course they come and go, but most neighborhoods are very stable. Actually N. Virginia/Arlington is where you see lots of turnover related to administration rollover, not so much DC proper.

Spicy Water is my new favorite place. Everything is really good.