Resonant Rattle

[Mulletwagon]

At about 45 mph I get a low pitch rattle, more like a loud growl, that comes and goes. Sounds like a bad water pump bearing, but a new pump did not fix the problem. Can't find any loose panels or worn suspension joints that could be vibrating. Wondering if it may be timing gear related. Previous owner contracted a high dollar rebuild so suspect a worn engine component is not likely. Engine has about 1500 miles since rebuild. Welcome theories.