You will likely find that a new computer case is not as easy as you might think. HP uses proprietary connections on the motherboard for power supply front panel and some rear panel connectors. In short, a real headache to cobble together.
The most noise generated from any computer comes from the cooling fans. I have found that switching the cooling fans from OEM to Noctua specialty fans makes a huge difference. They make them for most size applications but are not priced cheap. You get what you pay for. I have gone so far as to replace the case fan, CPU fan, also the PSU fan with Noctua fans just to quiet the computer. The secret is slow spin custom pitch on the blades to produce high air flow minimum noise.
The HP Z series workstation was never intended to be a bedroom quiet computer but more for a lab environment where noise is the least consideration over efficiency. The HP Z series computers are designed to perform for long periods of time without interruption. They are very good but also very noisy.
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