i just spent 4 hours on the road after fixing my rattling passenger seat... the first hour was the best, finally no more rattle!!! but the next 3 hours was agony, as it brought the engine mount clanging noise and this mysterious humming sound to the forefront
i've gotten hold of a voice recorder and will try to use it tomorrow to record the sound. one
very interesting thing that i noticed, is when i run over ruts and potholes in the road, the 'thud' sound i'm hearing is exactly the same frequency as the humming sound. and because of its aural characteristics and that it's always a single pitch, i think it's just some panel or part that's resonating... which is very fixable.
i'm actually thinking, this might be a positive sign of my sound deadening... it's reduced resonance noises from so many panels that there are only a few distinct ones left... but unfortunately because they are distinct, they stand out like sore thumbs

same kinda phenomenon as quietening your pc... eliminate your noisest part, and the next noisiest part becomes your biggest problem.