Prevent Sleep – How to Stop Sleep Mode in Scripts on Windows 7

Based on that description I would guess a fan, most likely if you just open up your computer, clean it and check that everything is securely in place it will go away.

You having put it into sleep mode is likely a coincidence.

Event 42 occurs too late (at the end of the process, no time to do anything useful) because it is intended as a "documentation" for subsequent diagnostics, not a notification before things happen. (I don't understand how could a MSFT moderator advice it?)

Unfortunately I haven't seen a working solution to your problem, but I'm sure that Windows does provide a before-event notification, although not in the Event log. This means that you have to create/use an always-running background application (~ a "daemon") that catches this pre-standby notification and then does whatever you like.
I'm not aware of an existing application that can do this, neither I know the relevant Windows APIs; but until a better answer is provided, here are some starting points that may help you in working out a solution:

• The following MSDN pages seem to lead to information about the notification itself: http://msdn.microsoft.com/en-us/library/windows/desktop/aa373233
  http://msdn.microsoft.com/en-us/library/ms703398.aspx#handling_sleep_transitions
  http://msdn.microsoft.com/en-us/library/ms701723.aspx
• If you don't want to dig into C++, then the start_power_monitor command of the TWAPI extension of Tcl/Tk looks promising (see the apmsuspend event).
  Tcl/Tk is not a complex product requiring installation and configuration, but rather a plain, very powerful executable that, when coupled with the single TWAPI .dll, competes with PowerShell (with much lighter footprint) (Although surely PowerShell could cope with the problem, too, just I don't know the relevant commands/packages.)

UPDATE:

There's no way to “hold off the sleep event” because Microsoft has discontinued PBT_APMQUERYSUSPEND that allowed it in XP and Vista. Instead, now says that you cannot prevent the user from putting the computer to sleep:

Applications should respect that the user expects a certain behavior when they close the lid on their laptop or press the power button.

Only the PBT_APMSUSPEND event remained, after which

The system allows approximately two seconds for an application to handle this notification.