Why do older fluorescent lights make a plinking noise when they are turned on?

Audible noise from a fluorescent light is likely due to a loose or faulty ballast. The ballast is an electrical device inside the lighting fixture that supplies the proper starting and operating power to the lamp.

Sometimes, changing a lamp loosens the ballast mounting or other parts of the fixture and the slight vibration from the ballast during normal operation causes a buzzing or hum. The volume of the sound may change - louder or softer - as the fixture warms up. You might try tightening any screws or loose fixture parts to see if this alleviates the noise problem. Checking the ballast mounting screws is a little more complicated since the fixture has to be opened, but that would be the next step.

Be sure that anyone servicing a lamp, ballast, or fixture first turns off the electricity to avoid the chance of electrical shock.

Noise can also be an indication that the ballast is about to fail. Ballasts normally last 10-20 years, but often become noisy just before they fail. If the light goes out and then comes back on unexpectedly after initially operating for a while, it means that a thermal switch inside the ballast has opened and closed - another indication that the ballast is at its end-of-life.

https://www.quora.com/Why-does-a-fluorescent-light-bulb-make-a-ping-sound-when-its-turned-on-as-opposed-to-an-incandescent-bulb

Older fluorescent fixtures used a "preheat system," which featured a bimetallic starter (the small, round, silver piece). Inside the starter is a bimetallic switch which "pings" when energized. Newer fluorescent systems, such as the "preheat" or "rapid start," are rendering the "ping" a thing of the past.

https://en.wikipedia.org/wiki/Fluorescent_lamp#Starting