What the heck is that noise? When I parked my Vanagon two days ago, it was running as quiet as a (loud and very large) mouse. Now it sounds like a tea kettle!
Welcome to another quirky (and common) Vanagon experience.
Up to and including the 1977 model, all VWs required regularly scheduled valve adjustments. In 1977, however, VW decided that it was time to join the 20th century and added an automatically adjusting valve system to the VW Bus. The difference between an engine with adjustable valves and non-adjustable valves is in the valve "lifter," which is also referred to as a "tappet" or "cam follower." So, VW added hydraulic lifters to the VW Bus beginning with the 1978 model. The same exact lifter used in the 1978 Bus is used in all Vanagons, both air and water-cooled waterboxer engines. Engines fitted with hydraulic lifters are commonly referred to as "hydraulic valve engines”, but it is actually just the lifter that is hydraulic. From the very beginning the hydraulic lifters were problematic—not in a catastrophic sense, but definitely in the "nuisance" category.
When any engine is shut down, some of the valves stay in the open position. This is inevitable in all engines; in whatever position the engine happens to stop, some of the valves remain open. On engines fitted with hydraulic lifters, the valve spring (on those valves that happen to be open) slowly but surely squeezes all the oil out of the hydraulic lifter, causing it to "flatten out." On most engine designs, when you fire up the motor, the lifter pumps back up within a second or two. Only the most trained ear will even notice this. On the VW flat-four engine—because of its peculiar design and rather minimal oil pump (VW made no changes to the pump or lubrication system, at all)—the process of pumping up can take several minutes. You can sit there and let the engine idle until the cows come home... but that won't normally fix the issue. The engine needs to be run at an elevated speed to build up oil pressure, usually for several minutes. The best thing to do is just ignore it and drive away, and the clacking will eventually disappear.
Note: We are not suggesting that you ALWAYS ignore a clattering sound emanating from your engine. This advice is only for this particular scenario: When you last drove your Vanagon (or Bus with hydraulic lifters), you're SURE the engine was running and sounding normal, right? And you are SURE no one removed the oil drain plug and let all the engine oil run out (look for a huge puddle under your vehicle). If a clattering noise ever just starts up while you're driving, it is probably NOT a "flat" lifter. STOP right away and check it out!