Get a Dell T5510 for example. It makes the same amount of noise whether working at full load or sitting idle. I had to get used to the absense of noise in my office after I bought it. And it is not just about being quiet but also maintain low noise while working at full load AND keeping the temperatures low at all the places with hot components. Standard casings and standard motherboards are sub-optimal by definition.
Those are baseline goals when building a properly quiet PC. There's nothing about standard casings and standard motherboards that make them suboptimal by definition. Choosing from a vast library of available parts means one can emphasize exactly those qualities most valued instead of a one size fits all. HP obviously isn't going to drop $100 on a single cooler for anything consumer or office grade whereas the sky is the limit when building your own. There is obviously also more room to mess something up which is where prebuilts come in with a reasonably balanced package without too much fuss or effort spent. Though it should be noted prebuilts aren't always flawless either.
I have to agree with troll-boy here.
I've worked with PC's and computers since the 70's and I've seen good and bad cooling in everything from my own units to factory units by all the manufacturers.
You people wouldn't believe the things I've seen,
I've seen HP factory servers on fire off the shoulder of Orion ... ok, slight exaggeration
I don't get why people assume that because hardware was made by HP, Dell or IBM that it's in some way perfect. Commercial companies build to a price and if the price won't allow decent quality or design, then you won't get it. End of story.
This workstation is a 8 core i7 with 32GB of ram and 12 TB of ZFS Raidz running FreeBSD.
It's in a brilliantly designed Enthoo Pro case and the CPU is water cooled. the box is sitting next to my left shoulder right now, and I can't hear a sound except a small buzz when a drive decides to work hard. This unit with the current hardware has survived a 46 degree C (114.8F) summer here in Australia and is not in a air conditioned area, but rather on a workshop bench in my steel factory shed.
I built this unit in 2012 using the latest tech at the time and I still don't need or want anything more. I probably paid around $2000 for all the parts, but can't remember exactly as it was nearly 8 years ago. However it has run 24/7 since the day it was built, it *never* gets turned off.
The Enthoo Pro case has air 'strainers' and is built for easy maintenance and cleaning. I open it up about once a year to clean the filters and vacuum out any dust. I live in a really dusty area and there is a sand blasting company down the road (their grit is *everywhere*), but this Enthoo case is like a clean room inside when I look.
My point is that money and skill *always* beats commercial offerings in PC's in every way you can afford, providing you have the skill.