Sounds like starter motor sticking OR slightly flat battery? Give the starter a tap with something heavy, see what happens.

there is a very old photo of my grandmother tapping the starter moter of her old car (might have been a morris minner) with the heal of her shoe and she had to do it every time she wonted it to start and did so for 13 years
mind you she had know idea why she had to, my grandfather just told her thats what you do so she did it

Aye. solenoid is sticking on the starter. Remove it, strip and clean it and chances are that will sort it out. Or maybe they're ten a penny for your engine, I don't know what you run.
Consequently I don't know where exactly it is lol. Just look at the engine block, it will be very visible and bolted to the side as it obviously needs to make contact with the flywheel inside of the bell housing.

Very similar to this mate:

(that's not a Volvo one, but they all look basically the same.)