Several owners have reported similar occurrences. The forward collision sensors spot a potential hazard and respond with either:
1. A warning chime if the car's speed is above a certain level (around 15mph I think - handbook is specific about this somewhere), or
2. The brakes start to prime (this is the scrunching sound heard) in readiness for activating the brakes at low speeds instead of giving the audible warning.
I believe the consensus is that this is "normal for the car". My dealer reconfigured the forward collision sensors and it seemed to improve things for a short while. Until, a few months ago with nothing on the road ahead of me, nor on the oncoming side of the road, nor any pedestrians/cyclists etc, the car activated the brakes and brought the car to a complete stop. I was doing less than 15mph at the time. A few weeks later the brakes activated again in very heavy traffic and a taxi behind nearly shunted me.
Again, the car went in to the dealer who again reconfigured the forward collision sensors, without ever really giving an explanation of why this would happen. So far the car is back to the more predictable scrunch (although not as loud as it originally was) when it spots cyclists or similar in the road ahead.
Hey ho.