I have been using a second-hand TRUE commercial freezer for almost 13 years for my home catering business. It has been making a buzzing noise since yesterday.
How can I identify the source of the problem?
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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 also be able to feel excessive vibration by putting your fingers on different parts of the freezer.
Look for broken parts.
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.