Overheating I turn the motor off I can hear a hissing noise

If the vehicle is overheating, we need to address that first however, I suspect the hissing noise is related.

I suspect you have an air pocket, or you are low on coolant. So, we need to start with this guide that will walk you through overheating issues and how to pressure test the system to check for leaks:

https://www.2carpros.com/articles/symptoms-of-an-overheating-engine

https://www.2carpros.com/articles/radiator-pressure-test

Then here is a guide that will help with flushing the coolant as I suspect that is where we will need to go:

https://www.2carpros.com/articles/coolant-flush-and-refill-all-cars

Lastly, if you find no external leaks then we need to check the head gasket.

https://www.2carpros.com/articles/head-gasket-blown-test

If we fix the coolant/overheating issue, then I suspect the hissing will stop as well.

Please run through this and let me know what you find. Thanks