My well used electric trimmer suddenly had an issue with it spinning slowly and the motor noise changing to a whirring sound.  I had recently replaced a worn out spool cap (bought a 2-pack with a spare).

Taking it apart was a bit tricky.  You need to remove the lower guard first.  It is held in place by one screw and a wide flat locking tab which needs to be pryed free  with a flat screwdriver.  There are 4 more screws on the top of the guard that hold an air duct from the motor.

With the guard removed, then it’s just 12 phillips screws holding the two halves together.  A spring may pop out which comes from the opposite side of the release button that extends the handle.

I noticed quite a bit of gunk in the gear assembly and also noticed the inner teeth of the plastic gear were worn.  The whirring I was hearing was the gear spinning on it’s own and not catching the inner teeth.  The air intake was also pretty clogged with grass so it would have been a good idea to clean it regularly to provide good air flow to cool the motor.

Probably time to buy a new trimmer and maybe upgrade to a cordless one.