A 1U chassis stuffed with high power dissipation components will be insanely loud like a jet engine. But it's easy to avoid that. The key for quiet is to use 4U chassis for anything that requires significant cooling so it has large fans.
On my home rack I have three 1U computers, two are Atom-based (fanless with SSD, so zero noise) and one is a Celeron-based with slow fans (about as loud as my MacBook). The larger machine is a 4U with larger fans, so same sound level as any tower case (a 4U is literally a tower case sideways, with rack mounting tabs).
A 1U chassis stuffed with high power dissipation components will be insanely loud like a jet engine. But it's easy to avoid that. The key for quiet is to use 4U chassis for anything that requires significant cooling so it has large fans.
On my home rack I have three 1U computers, two are Atom-based (fanless with SSD, so zero noise) and one is a Celeron-based with slow fans (about as loud as my MacBook). The larger machine is a 4U with larger fans, so same sound level as any tower case (a 4U is literally a tower case sideways, with rack mounting tabs).