Not sure whether you'd consider it natural, but I first noticed this with the sound of running taps. The bathroom was next door to my bedroom and I'd often get up and turn the taps off if someone left the bath running... not sure if anyone ever noticed!

Not sure if it's the same for everyone, but for me it's caused by an over-focus on the sound. Once I've noticed it, it's like the sound gets louder and louder...

[edit] I'm fairly sure my Dad was on the spectrum, and the sounds young kids make would make him very distressed, shouting or banging etc. so I don't think it's synthetic vs natural sounds.