I'm thinking of slowly pushing the car into the pond.
What makes you want to push a car into the pond?
I'm wondering how loud the splash will be
There's absolutely no reason to push a car anywhere near water, and you need to be careful when you're near it. You could drown if you play around in the water too much and damaging a car could be a serious felony. Just leave this all alone and don't do it.
Fine, but that's not what I had in mind.
If you are curious about splashing water there are many youtube videos of ASMR water. It isn't right to damage a waterway and poison the environment to just hear a temporary sound.