That doesn't answer the "why" it must be constant instead of every x minutes. That just states that it does communicate with itself constantly.

So I'm guessing your answer is that there is no way to schedule these self-communications.

Is that on a wish list for future versions?

Quote from link given above:
...that you can set BOINC's parts in the exclusion zone in ZA these days. Then it won't register on ZA's system tray icon.

How do I do that? I'm using ZA 6.5.731.000.

Quote from link given above:
Install BOINC as a service and only use the Manager when you need to check on it.

Is there a way to do that without re-installing? Or do I need to re-install?

I don't use the windows version, but with the Linux version the constant traffic that shows up is the manager (the graphical window) talking to the client. What it's doing is updating the information you see on the screen (time left, etc.), which is why it can't just do that ever 15 minutes.

On Linux you can just close the manager (leaving the daemon running) and the traffic disappears. Try closing as much stuff as possible without actually exiting BOINC (is there a system tray icon? try to disable it).