Metallic rattling between 2-5K rpm
 

Recently bought a brz and it had no sounds for the first few days but after my mate revved it hard on a cold start yesterday its been making these rattling sounds between 2 and 5k rpm, probably at its loudest at 4.5k rpm.
My engine is stock and I just have an aftermarket axle back muffler and exhaust.
I can clearly still hear the engine but it just rattles in addition to the normal sound, its not extremely loud. So its not engine knock right? can someone point me in the right direction

I would suggest trying to locate the noise from outside the car, hood up. Even being able to identify if it's coming from engine bay, back of the car, somewhere in the middle will help you narrow down what could be causing it. I wouldn't rule out something metal cracking from quick temperature change, though. Also, I would suggest with anything aftermarket to double-check the installation and make sure an exhaust hanger didn't come loose.

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Every Subaru I've had or been associated with had a heat shield rattle at some point.

Never rev is hard on a cold start. Shame on him for doing that.

Does it do it in neutral? If not I may have a solution for you. I had this too. It was the inside nut of the valve stem that got loose. It was rattling around inside the wheel at around low speeds but then when I got up to faster speeds it dissipated. Annoying bugger of an issue.