Unusual traffic from your computer network

[sims]

Hi,

First of all i don't know the thread is in correct session .

Users are often getting the message from google and they are forced to enter captcha  "Unusual traffic from your computer network", 

When i capture the traffic from the moment user hit google.com , the software captured around 500 frames in both directions . 

So the questions from fortigate side  ,how do i check which source is sending unwanted traffic to google . 

and  how to limit if they are sending huge amount of packets .

Please help

Thanks