Tim, looking at the floorplan I would have to think it is coming from the bathroom - probably something came loose during transit with no inspection on either end. Probably under the sink. Have you got access to the water lines under there? If it doesn't appear so and there is a panel or something, it should come out. Here's a link for the benefit of others;
Edit: at full pressure I'm sure you had pressure in your water lines even if the low point drain was open. I assume you did close it off?
Have you checked your water heater to make sure it has the plug/anode rod in it? When you get this sorted out you will need to check the water heater to see if it is in bypass mode; which it probably is.