X velocity changes when only moving forward

Godot Version

4.3

Question

I have a simple player movement script built off the default one. However the default doesn’t have a section for looking around based on the mouse’s movement. I made my own and used 2 nodes, camera3d and node3d (named Head). Head is used for x rotation and cam for y rotation. When I move forward by only pressing the W button, the x velocity also changes by a small amount, causing me to move left or right slightly. Strangely this only happens when I am looking forward. If I look directly up or down the problem goes away. This is the Player’s code.

extends CharacterBody3D


var speed = 7.0
const JUMP_VELOCITY = 300
var friction = 0.3
var sens = 0.005

@onready var head: Node3D = $Head
@onready var cam: Camera3D = $Head/PlayerCam
@onready var col: CollisionShape3D = $PlayerCol

func _unhandled_input(event: InputEvent) -> void:
	if event is InputEventMouseMotion:
		cam.rotate_x(-event.relative.y * sens)
		head.rotate_y(-event.relative.x * sens)
		cam.rotation.x = clamp(cam.rotation.x,deg_to_rad(-80),deg_to_rad(80))

func _physics_process(delta: float) -> void:
	# Add the gravity.
	if not is_on_floor():
		velocity += get_gravity() * delta

	# Handle jump.
	if Input.is_action_just_pressed("jump") and is_on_floor():
		velocity.y += JUMP_VELOCITY * delta

	# Get the input direction and handle the movement/deceleration.
	# As good practice, you should replace UI actions with custom gameplay actions.
	var input_dir := Input.get_vector("left", "right", "up", "down")
	var direction := (head.transform.basis * Vector3(input_dir.x, 0, input_dir.y)).normalized()
	if direction:
		velocity.x = move_toward(velocity.x,direction.x * speed,friction)
		velocity.z = move_toward(velocity.z,direction.z * speed,friction)
	else:
		velocity.x = move_toward(velocity.x, 0, friction)
		velocity.z = move_toward(velocity.z, 0, friction)
	print(velocity.x)
	move_and_slide()