Water Sound Behind Dash

Hey Guys, I have been having a sloshing/water sound coming from what sounds like behind the dash. I took it to certified service and they couldn't replicate it and said the A/C hose was draining fine. Sure enough when I drove away and went over a speed hump I could hear it once more. Any other ideas on what it could be? Thank you! 2016 Chevy Impala 2LT

Check the cowl drains as well.