static final sinePeriod = 2 * pi;
double _endValue = 0;

if (episode.percentDownloaded == 1 && !episode.hasNotified){
  _endValue = sinePeriod;
  episode.downloadNotified();
}

TweenAnimationBuilder(
  tween: Tween<double>(begin:0, end:_endValue),
  duration: Duration(milliseconds: 200),
  child: child,
  builder: (_, double value, Widget child){
    double offset = sin(value);
    return Transform.translate(
      offset: Offset(0, offset * 2),
      child: child,
    );
  }
)