Hi
how long does it run at high speed?
Are you playing intensive games or watching videos at the time?
If you have Win 10 installed, when it occurs, right click on the Taskbar and click on Task Manager.
Select Processes > CPU and check the CPU % and also the name of the process that is using the highest CPU %.
Also check in Event Manager for any Critical, Error or Warning events that have a number of the same event IDs and that may have occurred at the time that the fan is running at high speed.
To get to Event Viewer, right click Windows Start button, left side of Taskbar and click on Event Viewer.
This is maybe a start to finding out what may be happening.