... and sometimes I am a total dork Occam. 
....
Well, I guess I should be flattered that my name can be used to describe the extent of dorkiness
Hey Ray,
Transverse mode noise, aka normal mode noise, is simply noise that is not common (in both phase and amplitude) to 2 lines in signal/power transmission. In a transformer, if the same noise/signal is presented on both ends of a winding, their imposed flux is opposite and cancels. This is why signal transformers are often used at the input of components; it exhibits very high common mode noise rejection.
But the purpose of a transformer is to 'transform' voltage and current that
is normal/transverse. A transformer has a hard time distinguishing between what is noise and what is signal. Toroidal transformers have many advantages. They are compact, efficient, and have minimal radiated fields. But because the primary and secondary are typically wound over each other, they have large interwinding capacitive coupling, and they pass noise between. [this capacitive coupling also compromises common mode rejection] The fix for toroids is to impose a grounded interwinding shield between the primary and secondaries. This provides a capacitive shunt to ground for noise. As line noise is typically high frequency in nature, the impedance of the shield's capacitve shunt to ground lowers in impedance with increasing frequency, it does exactly what we want it to do, minimally affect the signal, but attenuate noise.
Other types of transformers can also use interwinding shields, or use physical separation of the windings to limit capacitive coupling.
This explanation is somewhat simplified, but sufficiently boring......
