I went through this.
Couldn't find any coolant leaks or stains anywhere to indicate where 1 gallon a week was going.
The exhaust did sound different under hard acceleration with a load (gurgling sound). No other symptoms such as power loss or white smoke.
It turned out to be a bad EGR cooler and head gaskets(so the dealer said, I think they pulled the heads to check the pistons for fuel spray patterns).