I don’t know if you have ever put your ear close enough to a T5 or any other fluorescent grow light, but I have. Just recently reaching for the plants at the back of my indoors garden I realized that my T5 lights hums a little. Of course it wasn’t very loud lime a fly or a mosquito, but it still was there. So I decided to research this topic a bit more and figure out why does my T5 light buzzes.
By definitions T5 lights are very quiet, because the thing that could give out any noise a.k.a. the ballast of the fixture for T5 grow light fixtures has the smallest noise level of 20 to 24 decibels, meaning, that they produce the least amount of ambient noise that people could notice and therefore are extremely quiet. However they are not completely silent. In fact nothing is, most of the time we just don’t hear many of the quiet sounds that happen around us.
Electric ballasts that all T5 grow lights now are equipped with are revolutionary technology because the usual core-and-coil transformer that was in regular magnetic and hybrid ballasts has been replaced with electronic components that let the ballast work much more efficiently as well as much quieter, because even with vibrations the electronic components won’t produce a lot of noise. However, there are other factor, too, that can amplify the noise that the ballast makes causing you wonder why your T5 or other fluorescent grow light is buzzing.
One of these factors is the way the lamp is mounted. That where and how your T5 grow light is hung up will determine first how much vibration the light gets and therefore how much noise it makes, as well as how much this noise will be amplified by the room where the light is located. If for example you have hung your T5 lamp from the ceiling and you live on the lower floors of an apartment building, the vibrations that are made by other people walking on the floors above you can cause your fixtures ballast and other components to vibrate and you to hear it more distinctively. Similarly if your fixture is hung in mostly empty room or room that has good acoustic that can cause for you to hear the slight hum of it that much better.
Another thing that can affect that how loud you light is, is also where in the fixture its ballast is located. 
The location of ballast, too, will influence the amount of buzzing you hear because for example if the ballast is located somewhere in the middle of the T5 fixture and there are other fixture components around it that can cause the ballast to hum louder.
And lastly of coarse the buzzing of a T5 light will also be heard more loudly if there are loose parts in the fixture and around the ballast. Even if one screw has somehow come loose inside the fixture, every time the light receives any kind of vibrations be it a passing car or a slamming of a door that loose part will vibrate making noise. And if you combine the vibrations of the ballast and that loose detail the sound can seem even louder.
So if you want to have as quiet of a T5 light as possible make sure that it the electronic ballast of it is healthy, that the light is properly mounted to the wall or the ceiling and it receives as little vibrations as possible and lastly also make sure there are no loose parts inside of the fixture that could amplify the noise that your light makes.
