[Anthony]

Well, this is as they say, where the plot thickens.

So now I'm thinking I need to add insulation, so I drill a hole 2" diameter with a hole saw to see what's what, where fireblocking is, etc.

Turns out, there's already insulation in the wall. It looks like 2" to 3" thick bats attached to the far (neighbors) studs, with a 3" air gap until my drywall. Edit: upon further reflection, it looks like they put bats designed for 2x4 cavities inside a 2x6 wall -- that explains the amount of fiberglass and air gap). So now I'm thinking I can't blow insulation in, but there's obviously some damping in there.

Ugh, now where to look? I do appreciate all the help.

I'm finding this a great exercise. We're not going to be in this house forever and our next house is either going to be new construction or a renovation job -- so best to learn on the starter house.