JMO, the loud noise was probably her calling out to them like "Hey, I'm the person who called 911". The officers window was down, there was no reason for her to pound on the car when she could simply speak to get their attention.
Note, they refer to a "loud noise" not a bang or explosion or boom. They're trying to rationalize being startled simply by hearing her voice, but it would help their defense if they could drop a vague hint to make the public think she was raising a ruckus or acting "strangely". That's the usual justification LE uses when they kill innocent citizens: "they were behaving suspiciously", "They jumped or pushed back or moved, etc. and I feared for my life".
Works really well when you're not talking to investigators. You can leak vague and incriminating information about the victim that leaves much to the public's imagination. Tainting the potential jury pool. I just get the feeling Noor is a real passive-aggressive, manipulative person.