while engine running a spark comes from crankshalft pully every 15 or 20 seconds

There is a bolt that holds the lower timing cover on that has backed out and is touching the crank balancer!! No problem at all to remove it and repair the problem. That's my guess. In any event the pulley comes off easy after removing the bolt! Pull it and see what up.

bring it in and get the mech. to look at it.

Verify your Belts, some belts make static and you see under car or with a test light put close to damper then you can see the spark.

The spark is jumping from the serpentine belt to the pulley...it's static electricity. To prove this, spray a little water on belt and the spark should stop.