I figured this out. Monterrey has Airplay Receiver listening on Port 5000. You can disable it in System Preferences => Sharing => AirPlay Receiver.
Here is more info.
https://developer.apple.com/forums/thread/682332
jlivet
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I'm developing locally using Postman and Jetbrains Rider IDE and am getting this also.
HTTP/1.1 403 Forbidden
Content-Length: 0
Server: AirTunes/566.30.1
I haven't found a resolution. It seems to happen intermittently. I'm going to update the OS with the latest update and try again.