Just installed natural gas fire pit and now getting a whistling sound
I'm assuming when you say "flex tube", you're talking about a flexible gas connector like this.



If so, the whistle could be caused by this connector. You should be able to find "Whistle-free", or "non-whistle" gas connectors, if you search the internet.  The other option, is to use rigid plumbing the whole way.

As d.george points out, you'll want to check the manufacturers documentation for the fire pit burner, to determine if the gas plumbing is properly sized.  1/2" is usually only good for a single appliance. You'll likely have problems if you run the BBQ, and the fire pit at the same time.