Radiator fan? Mine is quite loud when it is on high speed.
Mine has the 3.2L diesel is there any reports of manifold problems with that engine?TC wouldn't be doing it constantly, only at certain RPM in certain gears, and your RPM gauge would be jumping up 50-150 when it does it.
Maybe you have the broken exhaust manifold bolts that some people with ecoboost have reported? You should get out and walk around the van and listen where it's coming from next time it happens.Maybe pop the hood open and listen. Could be exhaust pipe or heat shield vibrating, too.