play car engine sound only when W key is pressed down?

I'm having a problem with my car engine sound. I can get the sound to work but when I stop pressing the W key the sound keeps playing. How do I fix this?

 using UnityEngine;
 using System.Collections;
 
 public class CarEngine : MonoBehaviour {
 
     // Use this for initialization
     void Start () {
     
     }
     
     // Update is called once per frame
     void Update () {
         if(Input.GetKeyDown(KeyCode.W))
         {
             GetComponent<AudioSource>().enabled = !GetComponent<AudioSource>().enabled;
         }
     
     }
 }