The second cause of noisy audio signals is known as the 'Pin 1 problem'.

This was described in the mid 1990s by Neil Muncy. The convention in balanced systems is to use either XLR (so called because X = Pin 1 = ground/screen, L = Pin 2 = 'Live', R = Pin 3 = 'Return') or Stereo Jack (where the sleeve acts as the ground connection) as input/output (Fig 9 & 10). So Pin 1 should be connected to the chassis ground as shown in Fig 1 so that noise currents are kept away from the audio. However, it turns out that a considerable proportion of pro audio gear actually connected pin 1 to the signal ground via the PCB, and thus allowed noise currents into the audio circuits as shown in Fig 11, where the pin 1 of unit 2 is connected to the unit electronics PCB rather than the chassis ground. Sometimes it is possible to see this by simply inspecting the connections inside a unit, but you can test for it by using a device called a 'hummer' (see the references at the end for the AES article which describes how to build this unit). By attaching the hummer to any pin 1 on the unit in question, and listening to the audio outputs, the presence of a 'hum' on the tested output confirms that there is a 'Pin 1 problem'.