It's audible if strong enough and you know which sound it is.
To make things VERY specific in how the sensor functions with the ECU software:
Using knock sensors, the car will decide if it's able to go higher on the ignition/fuel maps towards optimal setting of 100% learned octane value. 100% is the optimal spot the car should be at. Below that, the car isn't running optimally (but doesn't mean it's BROKEN, just not as good as it could be). Lower values indicate normal wear, poor/wrong fuel, or damage. That's why some guys run better on 93 octane, even though the GT only needed 91 when new. It's not an illusion every time, and there is a quantifiable way to check with an OBD scan tool (learned octane value) with live data display capabilities.
Octane learned value when high does optimal ignition timing and fuel use
When low, it pulls timing (less efficient, meaning you need more fuel and air to make equal power, including cruising) and generally richer ratios
Below 89c coolant temperature, it won't try to learn new values.
Learning happens ONLY in the 1500-3000rpm range.
To decide if it goes up or down, the car splits it into a high load and a low load mode.
Low load is below 160 load value (a complicated to compute value that looks at how much force the engine is putting out, basically, without being externally measured).
High load is 160+
At low load, if it detects 12.5%+ of max value sweep on the knock sensor for 0.6 seconds, it will lower the learned octane value. If it's below 12.5% for the 1.8 it will learn up the octane value.
At high load, if it sees 25%+ max sweep value it will trigger the learn down. Less than 25%, it can learn up. The engine naturally makes more noise and vibration under high load, which is why the value can't be 12.5%
Beyond 4000rpm, the car will look at knock enrichment. This is adding extra fuel beyond the typical values. I don't have that data on exact amounts of change in front of me.
