Object jitters when the scene starts

In my game, I am using a square with a rigidbody2d attached to it. I want the square to move along the y axis when the scene starts at a constant speed. The problem is that when the scene starts, the square moves a little, pauses for a split second, and then moves smoothly for the rest of the way. How would I keep the square from pausing at the start?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 
 public class AddForce : MonoBehaviour {
 
     // Use this for initialization
     public float thrust;
     public Rigidbody2D rb;
 
     void Start()
     {
         rb = GetComponent<Rigidbody2D>();
 
     }
 
     void FixedUpdate()
     {
         
     }
 
     void Update () {
         rb.velocity = new Vector2 (0, -1) * thrust;
     }
 }