HDD can cause reboots as well so look into getting it replaced. Buzzing sound from the HDD certainly isn't normal and probably just biding it's time.
If you haven't created your USB media to reinstall Win10 fresh you'd better do it now. Since you won't be able to use your previous Windows (7 or 8.1) key to install as it wont be valid for those Windows any more. Windows 10 gives a one month 'to try' then afterwards your key you upgraded with will be tied to it. Only be a problem if you didn't have another computer at least to set up the usb if the HDD went completely dead.
http://www.microsoft.com/en-us/software-download/windows10
If your bios is able to boot with USB then this will be easiest, Microsoft reckons a little over 3GB space but i'd have at least 4GB.
Run the tool, 32bit/64bit which ever bit your Win10 is. It will ask which bootable method to create (DVD or USB) and an option download the version you upgraded to, so get that part right with the Win10 version.
It will set up the usb for you and can reinstall Win10 whenever. Your Win7 key has been marked as upgraded to Win10 on their servers so when you go to enter key in the initial Win10 setup, skip that part. It will ask for Cdkey twice, skip both times. Once you're connected to the internet it will activate.
It wont recognize the win7/8.1 key anyway if you put it in, i tried