I bought a custom built PC years ago and specifically requested it be quiet, alas it wasn't quiet enough. What I finally did, bought a sheet of rubber and hung it over the fan outlet using tape, acted as a sound dampener. So it was a flap, with the fan running it swung away from outlet allowing the fan to breathe.
If you put your hand on the laptop and can feel the vibration then you need a new cpu fan. These replacement fans are from China. You can get one from Ebay or AliExpress. I did cpu fan change on one of my laptops. The first replacement giving the same problem after a few months. Only the second one from another supplier fixed the problem. If you do not have experience of putting together a PC, then get one from either Dell or HP. Any noise from either laptop or desktop, it's from fans. Changing old fans will fix the noise problem. If don't need mobility and have no space constraint then desktop is better. Generally, desktop last longer as it has better ventilation.
Electronic noises, any sort of buzzing or humming, especially from things that are supposed to be quiet, drive me nuts. I'm sure somebody has already mentioned that you should get longer cables, if you are going to stick the thing in the closet. You can order cables from Amazon and they aren't too expensive. I recommend you get 100 ft ones, that way you will have them if you need to use them for something else in the future.
A good set of noise canceling headphones will filter a lot of/all of the computer noise too and give you good sound and hearing for videos/tv via Bluetooth while working on your puter.
Nowadays, computers are using lots of power. previously we used < 0.5 kW. Now we easily use > 1kW. more power --> more heat generated--> more fans needed.
What brand and type of notebook is it? How many years are you using it? It could be that the cooling fan has become dirty and thus less efficient. Then it has to work harder to keep your computer at a reasonable temperature. Causing more noise. It could also be that you push your computer to its limit, with heavy loads on the CPU, causing it to create more heat. Investigate whether you can read the temperature and CPU load of your computer to determine what is going on.
My laptop is three year old, HP Pavilion with CPU Core i5-7200 U. When the trading program is running, the fan begins to make noises.
We don't know the configuration of his closet, it could be a large walk-in closet. The man said he wanted to place the unit in his closet, I'm fairly certain he wouldn't put it in there if it would be a safety hazard.