I have a few P4 boards lying around and a nice C2D intel board. Issue is all these machines have CPU fans that are loud and obnoxious.
Only fans I have found to be too loud are the HP SFF systems with the squirrel cage air ram that spin up and sound like a jet engine. But this only seems to occur on the 2.8Ghz and faster Pentium 4's that have the Hyperthreading for some reason and higher TDP. The 2.6Ghz HP SFF systems the company I worked for had were quite quiet.
If you have a normal socket 478 or 775 heatsink for your Pentium 4 and the Pentium 4 is adequately cooled it shouldnt be very loud. The only fans that were loud as far as these heatsinks go are the fans that are dying and start to groan and the case resonates and amplifies the sound or you have a high end gaming heatsink with heatpipes and its like a server heatsink with massive airflow and high RPMs intended for a hot blooded overclocked CPU etc.
Here is my list of systems I have gotten for free and have in dry storage:
Pentium 4 1.7Ghz 768MB PC-133Mhz RAM 40GB HDD
Pentium 4 2.0Ghz 1GB DDR 266Mhz RAM 60GB HDD
Pentium 4 2.4Ghz 1GB DDR 266Mhz RAM 80GB HDD
Pentium 4 2.66Ghz 1GB DDR 333Mhz RAM 80GB HDD
Pentium 4 2.8Ghz HT 1GB DDR 400Mhz RAM 160GB HDD Foxconn motherboard has 8x AGP slot available for video card but the motherboard has 1 swollen capacitor in the integrated sound circuit and a darkened area around a VRM that ran hot near the AGP slot "so I dont trust it".
Pentium 4 3.0Ghz HT 1GB DDR 400Mhz RAM 120GB HDD unfortunately no AGP or PCIE slot and PCI video card is limited its an eMachine. I have a Geforce 2 64MB PCI video card in it, but its a sad motherboard crippled by design to not have an AGP slot.
Celeron D 335 2.8Ghz 1GB DDR 400Mhz RAM 160GB HDD AGP 8x Geforce 6200 XFX brand
Athlon 64 3200+ 2.0Ghz 1GB DDR 400Mhz RAM No Hard Drive Gigabyte Gaming Motherboard supports SLI but only have a single Geforce 7600GT in single slot.
Athlon 64 x2 4400+ 2.2Ghz 1GB DDR 400Mhz RAM dual Geforce 6800GT video cards SLI No Hard Drive
My Daughters Core 2 Duo E4500 system is getting rebuilt and I gave her the Celeron D 335 2.8Ghz to use. It plays Minecraft fine with Geforce 6200 XFX video card with 256MB VRAM AGP 8x. Since she likes that computer and isnt complaining about performance, it is actually quite snappy performance for a Celeron D with Windows XP SP3, she might keep that system for a bit. She also likes to program with Scratch on it so she brings programs between school and home that she does in the MIT Scratch program.
As people toss out their old computers I generally get them and scrub their drives clean or give them the drive to take and destroy in which i end up with a system without a hard drive until I add one.