The 2U servers are a steal, but def not as modifiable as the original 4U units.
With the 4U servers you were looking at 120MM fans, and a standard desktop PSU. This set-up could be made to operate at very low volume levels, comparable to a Desktop.
With the 2u unit it is more difficult, because most of the PSU with enough power to drive all the 3.5inch drives, in a 2u form factor, will be much louder. Most 2u PSU's are designed for enterprise solutions, where server rooms are used, and noise is not a factor.
The Stock PSU's on either of these units could be modified to run quite, but you would need to get new fans, and the fans for 1u PSU's are generally 20mm which means they run at high RPM's and are LOUD.
I have a few of the 4U units, and have not modified them at all, as I keep them in a rack in my garage.
Happy to see so many people are finding these servers to be useful.