I have a complete Megan system with that Magna cat, and I have an exhaust leak/high pitch metal chirp noise too. Me and my grandpa looked for the source for a while, then said eff it. It kinda sounds cool after a while... is that my secret er side??
but to be honest, my AEM V2 makes a whistle sound when it starts sucking in air when you hit the gas, and inside the car I pretend it's a tiny little turbo because the sound follows the rpm band for like 2k
oh to answer your ? from earlier, I believe the cat came with it's own hardware and they fit directly in the Megan exhaust flanges on either side. Mainly hassle-free.