I narrowed things down this time to "Block known communcation channels used by attackers" in the AV section of the client. I was fine until I enabled that feature.
Yes Also confirmed as per above.
Forticlient was the issue for me as well.
The IGMP storm would occur after a new device connected to the network. i.e. an HP printer that was currently turned off, once the device was turned on, the IGMP storm would occur around 4-5 seconds later. The IGMP storm would however originate from my own desktop pc from that point forward. It took down the whole network!
Two Important notes
1) Once the flood started removing the HP printer would not correct the issue, I needed to remove my pc from the network to stop the flood.
2) Even though the flood was originating from my pc, the IP address being spammed in the flood was the IP address of the HP Printer (even when the HP Printer was not connected to the network anymore.)
Our FORTI-AP that was directly connected to the Fortigate and which is also on a different subnet went offline when this occurred.
As per above I disabled "Block known communication channels used by attackers" on the FortiClients and the problem has now gone.
I am using Windows 10 on the affected pc.
I spent 4 hours on this and wish that I had read this post first, could have saved me a lot of headache.