Why is the system unit making noise?

You should only vacuum the inside of the computer case with an Electrolux vacuum cleaner.
And to stop the fan from making noise, you need to disconnect it and put the computer case in the refrigerator to cool the processor. You can also surround it with ice... or pour cold water over it.

Lelya and I came up with another idea... so there's also the option of using a barrel of pickled cucumbers... but the barrel has to be big enough for the computer case to fit inside without any problems.