Adding noise to examine a circuit's performance
1) To induce white noise on a thermistor, you can parallel it with a solid carbon resistor at least ten times the thermistor's room temperature value. Solid carbon resistors create a lot of wideband noise.
2) To induce ripple into a power supply use the existing power transformer, or find one that outputs about 6 to 12vac. A wall-pack transformer would work as well. Connect one end to the regulator ground if it is an external transformer. Connect a 10uF non-polarized capacitor rated 3 times the voltage before the regulator, to the regulator input. If it is powered by a transformer use the same capacitor but use the transformer output before the rectifier diodes as a source for the ripple. These are simple low cost ways to inject noise. To increase the noise increase the capacitor value.