I had a similar noise once 2yrs ago.
Driving at 40mph and saw bunch of yoofs throw something at me...........BANG
Saw them run off laughing, pulled over ASAP but they were long gone.
Inspected the door expecting my heart to sink but luckily all I could see was a wet patch.
Must have been a ater bomb
However, I too have had a spring sap on my old car.
Was normal driving taking a corner at a lowly 15mph, no idea how these things happen