Project Boost Audio Issue

using UnityEngine;

public class Rocket : MonoBehaviour {

    Rigidbody rb;
    AudioSource rocket;

    void Start () {
        rb = GetComponent<Rigidbody>();
        rocket = GetComponent<AudioSource>();
    }

    void Update()
    {
        Thrust();
        Rotate();
    }

    private void Rotate()
    {
        rb.freezeRotation = true; //take control of rotation
        if (Input.GetKey(KeyCode.A))
        {
            transform.Rotate(Vector3.forward);
        }
        else if (Input.GetKey(KeyCode.D))
        {
            transform.Rotate(Vector3.back);
        }
        rb.freezeRotation = false; //resume physics control
    }

    private void Thrust()
    {
        if (Input.GetKey(KeyCode.Space))
        {
            rb.AddRelativeForce(Vector3.up);
            if (!rocket.isPlaying)
            {
                rocket.Play();
            }
        }
        else if (Input.GetKeyUp(KeyCode.Space))
        {
            rocket.Stop();
        }
    }
}