That sounds like an exhaust leak to me...a knocking sound, but the cause is actually escaping exhaust gas from a bad manifold gasket or bad seal against the head.
Because it only happens when cold, which is when exhaust leaks tend to happen, and because it doesn't go away with increased RPM (like a knock from low oil pressure would as the oil pressure increased), I would have them check for a loose spark plug, or an exhaust manifold leak.
Because it only happens when cold, which is when exhaust leaks tend to happen, and because it doesn't go away with increased RPM (like a knock from low oil pressure would as the oil pressure increased), I would have them check for a loose spark plug, or an exhaust manifold leak.