On older cars, like my '72 Dodge, the system voltage varied between 12 and 18 volts. But the dash instruments needed 5 volts. This was achieved with a clever "buzzer" circuit using an electromagnet and contacts. The circuit would open when it was above 5 volts and close when it was below. This created 5V, but was a noisy 5V.

Many people decided to improve this with a semiconductor voltage regulator, which would nail the output at 5V. But the instruments wouldn't work! The problem turned out to be the instruments relied on the noisy 5V to "unstick" the needles on the instruments.

So the electronics guys had to add a "noise" circuit to the voltage regulator circuit.

P.S. Watch an old aviation movie, where the pilot getting ready to fly would tap the instruments to unstick them.