Fan speeds are controllable via the BIOS in your HP 6300/8300 have you tried that first ?
Sometimes you'll get a refurb where the fans have a lot of hours on them and they get noisier.
They can be replaced with new HP fans or the Arctic 90 mm fans also work well and are quieter.
To find out which one is noisiest, disconnect the fans one by one while the PC is running to hear each one's noise level.
If you have the SFF for example, disconnect the front fan first, then then the rear exhaust and you'll be left with just the noise from the PSU fan and your graphics card.
The HP Elite and Pro lines are made to be run in noisy office and warehouse environments so HP didn't bother to include more expensive silent fans for home users that prefer little to no noise from fans.
Also see:
Hi, I have a HP ProDesk 600 G1 DM with an i3-4130T cpu and the onboard HD4000. Is this hackintoshable?
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