Check battery leads for tightness and corrosion. If all okay, then it is probably the solenoid contacts on the starter motor on the way out.
It will gradually get worse, and might end up with the starter motor staying engaged after the engine starts. That can cause a fire.

Just ask me how I know!!!!!!!!!

If you know what you are doing, you can pull the starter motor off and replace the contacts for less than $20, or a new starter for about $120 plus installation.

PK