Inducing Hum Deliberately in Analog Circuits
For electromagnetic/electrostatic noise, using a low voltage wall-wort power supply, voltage divide the power supply in half and attach the divided signal into a non-inverting op-amp. Use a 100kohm potentiometer connected to ground to adjust the amount of hum.

In general, the idea is to amplify the noise floor much higher than you would normally without any additional input. If you want SMPS noise, do the same with an SMPS power supply. The same goes for PWM outputs. 

As a side note, white noise can be generated by doing the same thing with the stable output of a voltage reference IC as the input to the 

For a really quick solution, use an MP3 player or computer with sample noise either recorded or found online. (This is most likely what I would do)

In the industry, a function generator tuned to the desired frequency or an arbitrary function generator with samples would be used.