Rattling valve train

I pulled into the shop to replace the seals and fuel filter basket in the injectors. Competed the job and started the engine just to discover it now has an awfully loud rattling noise that sounds like valve train rattle at cold startup, but it doesn't go away. It did not make this noise when I parked it. I thought something might have fallen into a cylinder while I had the spark plugs out, so I pulled the plugs out and went in with an inspection camera to search for debris. Nothing, so I inspected the valves and all look fine. Removed the valve covers to check the shims everything looks fine. Even double checked the timing marks and pulled the oil pan to inspect the oil and look at main and rod caps, again, nothing damaged or broken. So, I did an oil and filter change, start the engine and it rattles badly. I'm guessing the oil pump is clogged or went out when I shut the engine off. So how does an oil pump just quit working? Now that I think about it, that is what it sounds like, that awful cold start rattle noise until the oil gets there.