In my case the building was concrete and steel, likewise the roof, the exhaust was outside and the generator inside.. So I support the Original Poster's findings.
There have been many threads about this.. I'm not sure that "internal vibration" is necessary the issue though Internal vibrartion is transmitted to the outside by the engine block and assorted attached parts.
An interesting thing. I had a 1,000 watt Enclosed portable generator (Genrac 1000) in fact I still own it, I just do not have it any more (If you find one with WA8YXM scratched or painted on it please let me know)
Weight, fuel consumption and noise all abut equal to a Honda EU-2000i..
But a 1,000 OPEN FRAME.. Way, way, way louder.