try changing ( change outlined in red)
Code:
$('#post_list').append("<li class='content-sub-menu' id='part'"+counter+"><div class='article'><h4 class='article-header'><a class='article-header-link' href='#tips'>"+data.header+"</a></h4><p class='article-body'>"+data.article+"</p><img class='picture' src='"+data.image+"'/></div></li>" ) ;
to
Code:
$('#post_list').append("<li class='content-sub-menu' id='part'"+counter+" style='display:none;'><div class='article'><h4 class='article-header'><a class='article-header-link' href='#tips'>"+data.header+"</a></h4><p class='article-body'>"+data.article+"</p><img class='picture' src='"+data.image+"'/></div></li>" ) ;
basic problem is you are appending the new elements and they are not hidden so they will show immediately.