Generally fans on lower end cases are fairly cheap. I've had several new ones that make lots of noise new. You can call the manufacturers tech support and mess around tryig to get a replacemet or you can just order an inexpensive replacement fan. Cyberguys is where I typically buy my replacements.
Make sure the computer is sitting perfectly level. Wife's Dell computer was a bit noisy. I noticed it was on a shelf that was leaning a bit sideways. So I moved it and checked it with a small spirit level. Now it is running very quietly.
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