The ride doesn't have any onboard audio, but there are speakers placed throughout the track. You can hear the sound you noticed in this video from yesterday! You can hear it at 1:37 in.
The hiss before this sound effect was always there and is not a sound effect but the sound of the track switching. The hiss was always followed by a dull thud, but this is definitely a new sound effect to accompany the sound of the track switch!
Also, sometime recently they added an offboard sound effect as the trains hit the bottom of the main drop. I never saw this mentioned anywhere but it's definitely new-ish.