Doug, the problem is that people don't think your posts are serious if they don't involve at least one "mang" and possibly a "brutha" or two.
FWIW, looking in the furnace room if the basement is finished is a good start but I suggested the "tap tap thunk" as a secondary thing to try because it's easy enough to do. Problem is you can't always assume that all joists run in the same direction. At least in my house for example there's about a 4'x6' section in my living room where the builders all of a sudden changed directions perpendicular to the rest of them. It's actually a spot where I'd have wanted a tank if I wanted a tank on the main floor. Stellar, I know. I'm sure most houses are built with sanity but if I've learned ANYthing in life it's that you shouldn't assume people will have done things in a manner that makes sense to anyone but themselves.
Word. Er, I mean .. um .. Mang. Brutha! Fo shizzle or something.