How to make object moving forward while looping up and down

Hi, guy! I'm new to Unity. I've created this script to make object looping up and down. How can I move the object to the -x axis. Something like a bird tries to avoid a beam that moves up, down and movs toward to the bird. Thanks everyone.

 public class updowntest : MonoBehaviour {
 
 
     public float delta; 
     public float speed;
     private Vector3 startPos;
 
     void Start () {
         startPos = transform.position;
     }
     
     void Update () {
         Vector3 v = startPos;
         v.y += delta * Mathf.Sin(Time.time * speed);
         transform.position = v;
     }
 }