WooCommerce中的产品内容挂钩问题

I've a big problem with my code in my functions.php in Wordpress. There is something strange. It's working fine but every load I got a warning in my debug.log.

My code:

add_filter( 'the_content', 'add_id' );
function add_id( $content ) {

    global $post;

    echo $id = $post->ID;

    $content .= $id;


    return $content;
}

Log:

[17-Oct-2018 15:53:18 UTC] PHP Notice: Post was incorrectly called . Product features should not be accessed directly.

I've tried all known ways to get a product but every time I get this error. What si wrong here?