How do I identify and fix a buzzing noise in my commercial freezer?
DIY repair tips will depend, entirely, on what the source of the problem is, and will be an excellent 2nd question. This answer will provide suggestions for finding the buzz, since we can't help you fix it until we know what the problem is.
Here are some tips for finding the source of the buzzing noise:

As jsotola suggested in a comment, listening carefully may allow you to identify where the issue is.

You may have to put your ear very close to different parts of the freezer.
You may have to stick your head inside to see if you can identify where it's coming from.
It's possible that something was put in the freezer with a stiff but flexible piece sticking out, and that protrusion was pushed against the inside wall of freezer just hard enough to buzz from the general vibrations of the freezer running.


You may also be able to feel excessive vibration by putting your fingers on different parts of the freezer.

Be careful, the coils can be hot!
Touching different parts might be enough to damp the vibration and stop the noise which will also tell you where it's vibrating.


Look for broken parts.

If a piece of plastic has recently broken (due to age, most likely) the broken parts may be vibrating against each other when the compressor is running.



I'm sure there are other things you can do to locate the source of the buzzing - use some creativity and imagination if these suggestions don't help you pinpoint the issue.
Once you've identified the problem, if you're not able to figure out a solution on your own (move that box inside the freezer, if that's the problem, etc.) post a new question, with a complete description of where the buzzing is coming from (pictures always help!) and I'm sure you'll get good recommendations on how to fix it.