Character is bouncing infinitely after I implimented gravity

I am new to Unity. I used a tutorial from Single Sapling Games on youtube on how to make character movement. Everything was working until just after I implimented the gravity code, at which point the character began to bounce infinitely on the ground. Here is my code: using System.Collections; using System.Collections.Generic; using UnityEngine;

public class PlayerMovement : MonoBehaviour {

 //VARIABLES
 [SerializeField] private float moveSpeed;
 [SerializeField] private float walkSpeed;
 [SerializeField] private float runSpeed;

 private Vector3 moveDirection;
 private Vector3 velocity;
 //Gravity
 [SerializeField] private bool isGrounded;
 [SerializeField] private float groundCheckDistance;
 [SerializeField] private LayerMask groundMask;
 [SerializeField] private float gravity;



 //REFERENCES
 private CharacterController controller;

 private void Start()
 {
     controller = GetComponent<CharacterController>();

 }

 private void Update()
 {
     Move();

 }

 private void Move()
 {
     isGrounded = Physics.CheckSphere(transform.position, groundCheckDistance, groundMask);
     if(isGrounded && velocity.y <0)
     {
         velocity.y = 2f;
     }





     float moveZ = Input.GetAxis("Vertical");

     moveDirection = new Vector3(0, 0, moveZ);

     if (isGrounded)
     {


         if (moveDirection != Vector3.zero && !Input.GetKey(KeyCode.LeftShift))
         {
             //walk
             Walk();


         }
         else if (moveDirection != Vector3.zero && Input.GetKey(KeyCode.LeftShift))
         {
             //RUN
             Run();

         }
         else if (moveDirection == Vector3.zero)
         {
             //IDLE
             Idle();
         }
         moveDirection *= moveSpeed;

     }


     controller.Move(moveDirection * Time.deltaTime);
     velocity.y += gravity * Time.deltaTime;
     controller.Move(velocity * Time.deltaTime);
 
 }

 private void Idle()
 {

 }
 private void Walk()
 {
     moveSpeed = walkSpeed;

 }
 private void Run()
 {
     moveSpeed = runSpeed;
 }

}