My laptop fan is usually quiet, but has started runnning even at low CPU usage, what can i do?

Turn it off,then turn it back on,if it does it again,turn it off for a while,then turn it back on,that should fix it.

Monitor temperature of CPU with software (if possible) to determine if it is overheating and fan is properly speeding up.  Or it could be the fan is making noise and CPU is still cool.

Cleaning dust out of older laptop used on fabric, or in dusty house for a long time might help.

Most people block the input bottom grill with pants or skirt or bedspread.    CPU heats up fast.  Use only on a table or buy a cooling stand or pad with or without included fan to keep air flowing.

Laptop fan bearing might need a TINY drop of oil. 

I use a USB external fan box to extract heat from the exit grill.  Drops the heat considerably.  These are on Ebay for less than $15 (from China unfortunately).

I got some more just this week but it took more than a month from order.

Open the back cover of the laptop and start blowing the dust off the fan.

Possibly lots of dust in the fans. Blow it out/use an air canister or gently use a cotton bud if the vents are big enough.