Audio signals are waveforms. They alternate up and down, hence alternating current. When one side is up, the other side is down.
Common mode noise is just that, it's not a alternating waveform but continuous noise interference. This is why it resides on both sides at the same time period. It's like the white water between waves on the beach.
CMRR or common mode rejection ratio is the ability of a balanced electronic circuit to cancel out that noise while allowing the audio waveforms to pass. It is measured in decibels. Most opamp based line input stages offer about a -40 db CMRR spec. Match up the inputs and source resistance and you can up that 20 more db's. Match up the current gain of the input transistors and you can up that another 20 db's.
In mic preamps this is an important spec if you want to avoid picking up dimmer noise, rf and other noise sources. With a feeble signal, that noise is more noticable. Great designs do better than -90 db CMRR 20~20k hz. With the best input transformers CMRR is frequency dependent. Most are good at below 100 hz but fall off above that. The best Jensens only do -70 db CMRR at 20k hz.
It is also an important spec in mixer sum circuits. Some offer balanced bussing with -80 db CMRR when single ended sum designs (most Neve, Trident, MCI's etc.) offer no CMRR reduction on the mix buss.
You can test this with a cell phone. Stick it up on top of the mic preamps, hear anything? On my console with balanced bussing I can lay the cell phone on the master strip with no interference. Do your mic preamps or console do that?