iMac Mid 2011 - EFI Rom firmware automatic update - horn or beep sound - black screen?

Hi All

So I have a 21.5 Mid 2011 iMac.

Ref:

Second hand iMac from Uncle, all good condition hardly used. - I have removed the Bootcamp and tried to start the iMac in recovery mode to wipe the device and it refused to start in recovery mode. Gets stuck halfway through progress bar. So i started it in Internet recovery and this fully wiped the disk (disk utility) and then installed the original OSX Lion. (Had osx Yosemite on it when i got it and Windows 7).

Starts up fine now in macos Sierra after Lion apple/app store upgrade to macos Sierra.

Possible issue or query:

During the Internet recovery the screen went black and a 'horn or single tone beep' came out of the iMac and then a white screen with a apple logo and a round edge rectangle progress bar appeared. This completed and then the iMac booted to oxs Lion.

I then upgraded it via the Apple store to macos Sierra (not High Sierra as my CS3 photoshop wont work). This seemed fine but when I was away from it my wife said that it made the same sound again and went black and yet it upgraded to Mac OS Sierra fine. After updating my icloud and start up options I then went to the software updates which where 1.3 GB and left this running. Only for my wife to say it did that noise again and restarted. (same white screen with apple logo and rectangle progress bar).

Has it done various EFI or Firmware updates from Yosemite back to Lion back to Sierra then an update via updates at the end? Or is this a common error?

Its not the 3 beep Ram error or anything like that, and so far the iMac seems stable and hasn't crashed or died or overheated. I have copied 42Gb of itunes files, backed up via Time machine and installed office 365 and all is well, its been on for around 6 hours since the downgrade/upgrade.

Trying to find out this on Google or Youtube has only produced two videos and these look similar yet I don't remember seeing any firmware/efi warning screens, it just done it.

Any advice would be great. Can i check anything to see EFI update logs to see if the times were the same as the 'horn sounds'?

Thanks in advance.

Cheers All.