For the swinging I changed the angle based on acceleration/deceleration:
//In End Step event acceleration = hspeed - old_hspeed; image_angle = image_angle * 0.8 + 10 * acceleration; package.image_angle = image_angle * 2; //package and cart sprites have same size and origin point, so the rotation looks good old_hspeed = hspeed;
For the engine sound GML has some neat methods to manipulate gain and pitch:
//In Create event engine_sound = audio_play_sound(snd_engine,0,true); //In Step event audio_sound_gain(engine_sound,1 + 0.4 * abs(speed),0); audio_sound_pitch(engine_sound,0.5 + abs(speed)*0.1);