Come to think of it, I don't really know what causes the classic high-pitched shriek of a jet engine. Is it just from the high-speed flow of air through the engine, or is it a siren effect from blades on compressors or turbines, or what? The shrieking sound always seems to be quite smooth compared to the buzz, although it has a certain edge to it that seems more audible at low engine speeds.
As for the white-noise roar that one hears especially on turbojets, I've always assumed that it simply comes from the high-speed exhaust (?), particularly since it seems to be really obvious on pure turbojets and aircraft in afterburner.
Bet you won't find an easy answer to that one: it was begining to become a topic of research back in the 70's. Would imagine that there are now models of the 'source' generators but can't imagine it's simple. Interesting about the sheet lead on the 747 etc: implies it was the resonating fuselage which amplified the effect rather than the driving source being the direct problem itself.