I have noticed it only recently, a kind of loud buzz. It's not continuous but starts up periodically and then stops again. I've traced it to the computer itself. The noise is made by the case, which is vibrating, but obviously it must be picking that up from something inside. If I put a fingertip on the case, the noise stops but I can feel the vibration.

This happens when the machine is idling, so I don't think it's likely to be disk noise. It could be the cpu fan, I suppose, but why would that be so active when the cpu isn't doing much? There is no internal power supply, so no fan there. The power cable has a built-in transformer, like a laptop charger.

Over time, fans can go bad - bearings wear off, or simply it could be dust (sorry, I am not trying to say you have dirty computer); have seen that many times.

You can check CPU temps and fan RPM with sensors (lm_sensor package), you probably know this already.

Something to keep an eye on - temps and RPMs

Could be the fan or ventilation system is clogged with dirt. The fan is running faster due to not enough cooling air.

The dirt on the fan blades is causing it to be out of balance which results in the noise.

hazel,

Switch OFF the PC but leave the power lead connected, so that the PC is earthed.

Undo the two screws securing the left-hand side panel and remove the panel.

Do not touch anything you are unsure about.

Inspect the CPU fan and any other case fans for signs of dust. Carefully clean the blades if possible with a small artist's paintbrush or slightly moistened microfibre cloth.

Restart the PC with the side off and try to locate the source of the noise.

If it is as bad as you say, it should be easy to find.

The traditional way is to use a length of tubing or wooden dowel placed carefully near the item to be checked, with your ear at the other end.

Please do not put the dowel in the moving fan blades!!

Good luck.

From Hazel's description it isn't a typical ATX type case design since the computer is using a separate power adapter.

I am guessing more like a NUC or ultra compact PC type device.

With the power disconnected, you can check the CPU fan plus any other fan within the case by seeing if the blades rock - press opposing blades to see. If they rock, the bearings are shot and the best bet is to replace the fan.
Yup! A mechanic's stethoscope! A large screwdriver used to be the preferred device.
Curious about your system as I've never come across a desktop with a power brick. (It is a desktop, not a laptop?) What's the make and model?

Play Bonny!

I suspect that it is a Lenovo Thinkstation E50 or similar with an external power adapter:
https://kingworldnews.com/review-len...nkstation-e50/

I am sure that hazel will let us know in due course.

Could be, I make frequent errors in my assumptions...

hazel,

If you have a Lenovo E50, I believe that it only has a CPU fan, so perhaps you can tolerate the noise unless it becomes severe?

Otherwise it probably means a trip to your local computer shop, whenever that becomes allowable.

The CPU fan and heatsink removal is covered on page 33:
https://www.mrmemory.co.uk/downloads...ons/152361.pdf

Used item:
https://www.ebay.co.uk/itm/Lenovo-31...cAAOSwlfFdsyZg

The surfaces of the replacement heatsink and CPU must be thoroughly cleaned with isopropyl alcohol before applying thermal paste the size of a rice grain centrally on the CPU.

These iso swabs are cheap:
https://www.amazon.co.uk/Universal-U...A3P5ROKL5A1OLE

MX-4 thermal paste:
https://www.amazon.co.uk/ARCTIC-MX-4...0097556&sr=8-3

You may be lucky and find that the old fan can be removed from the heatsink without actually separating the heatsink from the CPU.

Alternatively, there is a 12 month warranty on this E50 at £112:
https://www.box.co.uk/(Grade-A)-Leno...p_2386717.html

I hope this info is useful.

Also, try not to use canned air to clean dust off fans. It's likely to force the dust into the fan bearings making the problem worse.

An old auto mechanics trick except they'd use a long screwdriver.

I've often used a cardboard tube from a roll of paper towels as a means of localizing the source of any noise. Hold it up to your ear and it move it close to whatever you think is the noise source. Sort of a cheap-n-dirty stethoscope.

If it's a noisy fan, you'll almost certainly need to replace it.

If it's a vibrating disk drive, tightening any mounting screws helps. Some cases that I have came with little silicone grommets that act as vibration absorbers for disk drives that help a lot for quieting noisy computers. You might be able to find some online if a disk is your noise source.

computer stores actually sell vibration damping material.

my arch machine #1 hums a lot too.

Yes, it's a either a Thinkstation or a Thinkcentre, whichever of the two has a SATA disk controller. I bought it second-hand a couple of years ago for about £80 and it's been a lovely little workhorse, more powerful than anything I had before. But I don't see anyone being willing to fix problems in such an old machine for a reasonable sum, especially now when all the shops are closed.

I'll transcribe all advice and see if I can carry it out. I don't want to touch anything in there but just looking shouldn't hurt.

It has always hummed a bit but this occasional loud humming is new.

Something else I just noticed. My computer case has a raised bit on the top in front, which is hollowed out at the back so that you can use it as a carrying handle. The power button is set into this with the disk light just beside it.

When the computer is just making its usual quiet hum, if I put my finger on this structure, just above the disk light, it sometimes makes this louder noise, which is actually more a buzz than a hum. Conversely, if it is buzzing, I can quiet it permanently by putting my hand on this ridge.

Note: I've just tarred up a couple of partitions for my monthly backup. This involves a lot of disk read/write action, but there has been very little buzzing. So I'm inclined to agree with the consensus on this post that it isn't disk-related. It probably is the cpu fan.

hazel,

There is no harm in contacting a local Harrow firm, PC Wizards Onsite for advice:
https://www.pcwizardsonsite.biz/

You could ask them the cost of fitting a CPU cooler and fan supplied by yourself.

Then you can then weigh the combined cost against buying a new old stock Lenovo E50 with a 12 month warranty.

I looked inside this morning. As I thought, there is no internal power unit; the only fan is the one on the cpu. It bears the legend Foxconn and it was indeed somewhat dusty. So I blew it clear (by mouth, not compressed air blower). It seems to be very firmly seated; I don't think there is any instability there.

When I started up, it ran smoothly with no buzz. So I shut down, closed up the machine and rebooted. At present it is only making the usual quiet hum, no buzzing.

I found the receipt from Computer Exchange and it turns out I have not had the machine as long as I thought. I bought it in January 2019. It was second hand but fully reconditioned so I would expect it to last for at least two years. My old HP lasted longer, though at the end, I had to nurse it along, setting the time by hand and patching my kernels to make ACPI work.