Ever since I purchased my new box a couple of years ago I've been concerned about the level of noise coming from the computer.
 
It's not enough to trouble a non-audio guy, but it's enough to trouble me!
 
I sent the pc back to Carillon who did the original build and they encased my HDD's in acoustic sleeves, saying this fixed the problem.
 
Well, when I got it back, to my ears there was no improvement.
 
Later last year, I sent it back again and they replaced my original system drive with a 1Tb SSD, but again, the noise levels didn't go down at all. All this leads me to think it's either the CPU fan or PSU fan.
 
Last weekend, I replaced the graphics card and took the opportunity to run it for several hours whilst it was out of the rack and with the covers off.
At first I thought it wasn't working because there was no appreciable noise whatsoever coming from it.
But, as soon as I put the cover back on, I could hear it again.
 
The noise doesn't start when booting up from cold, but gets louder over the course of an hour or so.
 
The cover itself is a fairly massive structure being made from 2mm steel - quite unlike what you'd get from an off the shelf computer, so I think this rules out vibration.
 
So, dear people, any ideas?

I'm trying to find a way to put up a shield that works.  My PC is in prolly the worst possible position for noise, but it's the best position for access to connection, etc.
 
Not the same as what you're doing exactly, but maybe there will be some good info.

Hopefully Bob!

I am not sure of your setup, but wondering if a portion of this is coming from resonance in the case itself. I have noticed the most noise comes from the case fans on my machine, but is easily blocked/isolated because it sits in a cabinet-like enclosure. That is probably the most helpful, but a very important note with this is to also make sure the machine can breathe so it doesn't overheat and cause more fan noise.
 
In a nutshell, dust can cause a fan to imbalance, so that is probably #1. Next the fan mounting should be snug (but not tight enough to strip screws) so the case/MB is mitigating any "wobble" further. From there, the case should be on rubber mounts or similar to dampen the vibration it is seeing; and lastly, the surface it is sitting on should also be devoid from possible resonance issues (either hard as a rock, or dampening like a carpet).
 
If fans blades are clean, you can do a quick check of fan mounting mounting screws, then test by putting the computer on a folded towel to check if it is passing vibration to the surface it is sitting on (or simply lift the computer while running if applicable).
 
Two side notes with "fans themselves" - If they have wobbled from dust imbalance and damaged bearings, cleaning them will help, but not solve the damage (they can still wobble). Also, fans have inherent noise based on speed (power consumption, hence noise, is 2^x the fan speed - double is 4 times more, triple fan speed is 8 times more). "Normal" fan speed is based on two things - how much heat it needs to remove (legitimate issue), and how the fans are set in BIOS. Some fans have a "Silent" BIOS setting, but be careful with this one, as you will want some active monitoring in place until you know how the system performs (mine are set to "standard").
 
Quick edit: If the computer breathes through the bottom (such as a laptop), do not leave it sitting on a towel which can plug the holes.

I would look at replacing the fan with fans advertised as "quiet" or " low noise". Over time blades might get slightly warped and start to vibrate which causes noise. Bearings can wear. Dust can be an issue too, but I can't imagine that Carillon wouldn't have cleaned the blades (maybe not).Manufacturers have several ways to get a lower noise with a fan. One way is to use a larger fan running at a lower rpm..this is highly effective. Another way is through bios. If you have a bios that supports fan speed you can have the fan ramp up or down according to cpu temp. Yet another way manufacturers get a lower speed is they add more blades which can move more air at less speed and in this way lower the noise.

Just some more information.
 
The entire computer is a rack mount affair which sits inside my rack.
There are vents on each side with about an inch clearance between the case and the side of the rack, and 2 more rear facing vents.
 
All fans are clean and free of dust.
 
I also have an earlier, more or less identical machine (my old XP32 box) which emits practically zero noise.
 
I'll have another play with it tonight and report back.
 
Thank you gents for your musings.

Damped or heavy cases don't prevent fan and drive noise getting out because the case has holes in it, though they will prevent case vibrations.

I've been building very quiet PCs since the mid 1990s. The ine in my sig has two fans, never overheats and my decibel meter often can't tell it from room background noise at one metre.

I suggest you look at Noctua's website for a big cooler that fits your cpu.

After that cooling is all about getting enough airflow to prevent overheating while keeping noise down.

Then look for quiet fans, the bigger the better. Noctua again are pretty good. However you also need to be running them at low voltage to keep the speed, therefore noise, down. There are a variety of fan speed controllers on the market, or cables which convert the usual 12v fan power points to 5 or 7v.

Your next problem is the video card fan. These can be incredibly noisy. I use a fanless card, but there are editors about for gpu card's BIOS settings which allow you to adjust the pre-set fan speed vs temperature curve. Lots of cards fan the fan up to full speed as soon as the card is doing more than idle. By adjusting the fan speed settings you can delay fan acceleration to when the card temperature actually calls for more cooling.

Finally there's the power supply fan. Another source of noise. Seagate make good fanless power supplies so long as you watch the total power drain. A case that puts the psu at the bottom can help keep the psu cool. Heat rises, and a psu at the top of a case is usually sitting in the hottest spot.

Then you need to test everything to find the lowest possible amount of fan noise. I do this by loading the cpu to 100% and keeping it there (prime95 is a useful tool for that) while I keep an eye on cpu temperature. So long as it stabilises for 15 minutes while below the cpu's maximum thermal rating I'm happy. This makes the cpu a bit hotter than normal at idle, but really pays off sound-wise. It's very rare for a cpu to hit a sustained 100% capacity, so running closer to the chip's rated temperature doesn't seem to cause a problem.

I do a similar thing with the gpu. I also check drive temperatures, though that's not an issue with SSDs. Suspending an HDD so it's isolated from the case, or even putting it in the case bottom on a layer of foam can do wonders for quieting them.

It's also worth trying case fans in different locations and voltages. The PC before this one had a single case fan which I found gave greatest cooling at lowest rpm when placed at the back of the case (where air usually exits) blowing into the case, with the top front 5.25" bay left empty and open for hot air to get out.

www.silentpcreview.com is a useful resource, especially the forums.

http://www.quietpc.com are a UK supplier of quiet parts, as are https://www.overclockers.co.uk. (no connection with either other than as a customer).

another option not yet mentioned is watercooling. I use 3 wc systems with longer tubing that allows me to place my rads where ever and easy cleaning. Sure made a difference from perpetual vacuum cleaner "On" mode to a much more tolerable ambience ( I still have a case fan for my hard drives- there are wc solutions for those too). There used to be in fact a passive wc system you could buy too, no fans. Heat build up on the rads with fans off just from the cpu is interesting imho. Complete self contained wc systems are available now, fairly reasonably priced with great longevity ( put those in the kids pcs a few years ago, no need to muck around with them since). Maybe worth looking into.

I have a Cooler Master Comos 1000. I choose it because it has sound insulation as well as 120 mm fans 4 of them. These are low noise fans yet move a lot air. The CPU fan is the loudest fan but is well contained by the case. This is not a inexpensive case but well worth the cost.
 
The overall noise from it is very low. A whisper. 

Out of curiosity Jonesey, have you done an SPL check? Noise unto itself is subjective, so is difficult to understand the situation.