I have ridden numerous times in first class and constantly hear the most bizzare airframe and aerodynamic groaning and vibrating that honestly makes myself, passengers and some crews quite nervous. I have talked to several crews, mechanics and none seem to be able to narrow the culprit. It is NOT the door seals that have been previously labeled as the source. This vibration is in the lower fuselage area just ahead of the wing root. Much like wheel well turbulence from missing brushes. It sounds a lot like flying around with the gear down or spoilers deployed. Recently rode in a SHUTTLE America plane, N634RP and it was the worst I have heard yet. Previous crews reported it was extremely loud at cruise and of course MX found no cause. Has Embrear been made aware of, or sent out any information to operators about this problem. By far the most bizarre sound I have heard in an airplane in 10 years of flying.