.loader{
    height: 600px;
    width: 3px;
    position: relative;
    overflow: hidden;
    background-color: #6b6b6b;
    margin: 40px auto;
}

.loader:before{
    display: block;
    position: absolute;
    content: "";
    top: -300px;
    width: 3px;
    height: 600px;
    background-color: white;
    animation: loading 2s  cubic-bezier(.645,.045,.355,1) infinite;
}

@keyframes loading {
    from {top: -300px; height: 0%;}
    25% {height: 150px}
    50%  {height: 300px;}
    75% {height: 450px}
    to   {top: 100%;
        height: 600px;}
}