Hi there!
I've already written in my U5-710 disassembly guide post that the purpose of opening the chassis was to locate the strange sound which I doubted to be coming out of fan or the HDD. It was too strange. I was hoping that eventually the new BIOS firmware could eliminate this, but nope. =(
Here's the synopsis:
When you start the AIO, there's almost no noise at all. Then, after a few (10-15) minutes of work, when it heats up, it starts to make crackling noise every 1,5 seconds. The periods vary. There is no certain pattern. When it heats up more, the sound disappears and does not show up again until the temperature is low enough to stop the chassis fan. In this case it might come up again. But usually doesn't.
What it turned out to be:
By default BIOS is set to control the fans' RPM. Otherwise you get the "jet plane taking off" noise. And no crackling problems at all 
)
I disassembled it, started from Live USB to eliminate the possible OS drivers effects, started AIDA64 stress test and watched the temperature rise. We have 2 fans on the back of the AIO. As you look at them the right one is recognized as CPU fan, the left one - is the chassis one. They are connected to separate connectors on the MB.
The CPU fan is rotating all the time gradually increasing its speed, but no discomfort, its noiseless while rotating at relatively slow speeds.
When CPU temperature exceeded somewhat around 53 the system decides to start the chassis fan and does this in very unusual way. As I see it:
- BIOS gives power to the chassis fan;
- it starts spinning, spins for 1 second;
- BIOS stops giving power (considering, maybe, that at given temp this is enough);
- the fan continues to spin inertially and while it hasn't yet stopped -
- BIOS again gives power and here is where we get the awful sound.
After some time as the temp rises on and the chassis fan starts to spin uninterruptedly and then no disturbances at all. It's almost silent.
You can see at the charts attached the whole thing. The chassis fan start-stop issue is clearly seen at the screenshot, then it comes stable (the blue-grey line). Compare it to the temp chart (below).
The main stress-test section is 19:07 - 19:18.
This is the video of the whole thing with sound:
https://youtu.be/uDtQ9TMxB34
I believe this is totally a soft/firmware (BIOS) bug. The only thing we need to eliminate the sound is to correct the voltage/power amount to the chassis fan so that it does not stop once started spinning and continues at the slowest speed. Either way - not starting and stopping all the time until reaching some border where it starts to spin continually.
The other way of solving this problem may be by connecting parallelly the chassis fan to the CPU fan. As both are quite silent it won't be any trouble, but this will involve some light surgery to the AIO PC.
I now address the ACER engineers - please tell if it is possible to solve the issue (which seems not so hard to eliminate) with BIOS upgrade only?
If so, please fix the bug in the next BIOS firmware upgrade which is totally necessary now.
Otherwise please tell me any way to solve the issue, as it annoys me seriously.
Anyone, any thoughts? Does someone have anything like it?
