How to add sound to a script

I have a health script, I want it to when it reaches 30 health, to play a looping heart beat sound. I am unsure of how to make the script do this. Any Suggestions?
Script:
using UnityEngine;
using System.Collections;

public class PlayerHealth : MonoBehaviour {
	public int maxHealth = 100;
	public int curHealth = 100;
	
	public float healthBarLength;
	
	// Use this for initialization
	void Start () {
		healthBarLength = Screen.width / 4;
	}
	
	// Update is called once per frame
	void Update () {
		AddjustCurrentHealth(0);
	}
	
	void OnGUI() {
		GUI.Box(new Rect(10, 10, healthBarLength, 20), curHealth + "/" + maxHealth);
	}

	public void AddjustCurrentHealth(int adj) {
		curHealth += adj;
		
		if(curHealth < 1)
			curHealth = 0;
		
		if(curHealth > maxHealth)
			curHealth = maxHealth;
		
		if(maxHealth < 1)
			maxHealth = 1;
		
		healthBarLength = (Screen.width / 4) * (curHealth / (float)maxHealth);
	}
}

Here, try something like this.

using UnityEngine;
using System.Collections;

public class Heartbeat : MonoBehaviour
{
	public AudioSource heartbeating; // Define in editor
	public int health;
	
	public void Update()
	{
		if(health <= 30)
		{
			if(!heartbeating.isPlaying)
			{
				heartbeating.Play();
			}
		}
		
		else
		{
			if(heartbeating.isPlaying)
			{
				heartbeating.Stop();
			}
		}
	}
}