If anyone knows, where does the first shockwave form on the 330 at the critical Mach number? Is it over the nose or the wing?
Difficult to tell unless one has access to the actual data. The velocity over the forward fuselage is mainly dependent on skin curvature with a bit of AOA influence. Supersonic flow usually starts on the roof just behind the glazing panels - deceleration back to subsonic through a shock wave a bit aft of that.
Well placed that any boundary layer noise generated by shock wave instability is transmitted to the crew!
Guessing a bit, but it is not impossible that the "rumble" is that sort of noise rather than full-blown wing buffet.