Hmm. How much of the actual hardware in a system *can* you audit then?
Off-the-shelves stuff is out of question: processors design at the hardware level is not something most of us can access, and I believe they also run some internal software over that, which is not accessible either. Same for the memory controller. *bridges are also out of the question. Mainline GPUs that can peek and poke through PCI, and most other extension cards almost always have some firmware parts that are closed sources.
Granted, there exist completely free and open sources alternatives to most of this stuff, but as seen by the recent compiler issue from microsoft, can you trust the people building the devices to make them up to spec? And them, do they use tools that are 100% certain to not change how things operates?
Sure, if all your hardware is open source and can be somehow 100% certified by yourself to follow the original design, and the OS you run on it is also 100% open source (this one is actually feasible obviously) and you built it yourself, using a compiler you 100% audited yourself, then you can trust everything.
But if not, there is no need to look for a hidden backdoor in the processor; it can as easily run a secret blob on the main CPU without telling the OS and still have access to all the hardware.