Well, the mystery is solved.

I called the dealer service center again and insisted on bringing the HCH in. When I reproduced the noise, the service rep immediately agreed that it wasn't normal. Then they did another test drive with something they called "electronic ears" set up around the engine. Using those, they were able to determine that the noise was coming from the front engine mount. They've ordered a new one and will be installing it next week. I'll let you all know whether that takes care of the problem.