Hello,
Valve train noises can come from many areas. Some fixes are simple and cheap, others can be costly and complex. First you need to identify the noise correctly to help diagnose the problem. Is is a tap, tap, tap, tap, tap etc noise? Is it a general clattering nose in general? Does it come and go or remain constan? Is it isolated to one area or does it sound like it is coming from one spot along the engine.
Start with the cheap and easy fixes first.
The valve cover hold down straps can be bent inward and touch the camshaft if the valve cover hold down bolts are tightened too much. Remove the valve cover and make sure you have plenty of clearance between the straps and the camshaft. This situation is hard to diagnose if you do not know what you are looking for. It can make a sound just like a loose valve adjustment and will drive you crazy trying to find it. There is a bit of info and photo in "the valve adjustment tour" on this subject.
Obviously loose valve adjustment will cause noise also. Check and set valve adjustment. If you find any adjusters way out of adjustment, remove the rocker arms and check their contact surface for flat wear spots. Worn rockers cannot be seen without removal. They will continue to make tapping noise no matter how many times they are adjusted. Rocker replacement will solve the issue as long as the camshaft is not worn also.
Worn ball stud adjusters can also cause valve adjustment to be inaccurate and non lasting. the ball studs can wear and form a "wear crown" on their tops instead of a round ball with flat. Replace the ball stud adjusters or grind the crown off flat to ball. This should improve adjustability and quiet the valves if this is the issue.
Improper valve geometry from numerous or severe valve and seat grinding can be an issue as Dan has mentioned many times. This is a bit rare, and difficult to diagnose so I would suggest looking at this last. In sever cases, thinner valve shim pads are used and the rocker arm contacts the valve spring retainer before contacting the shim pad. The result is an isolated tap from that particular valve. The fix could be simple or possibly a complex head removal.
Timing chain stretch or wear can cause a general clattering noise while running or under acceleration. The timing chain is made up of 134 links each having a hinged pin. These pins will wear slightly like any hinge pin from years of use. A minute amount of wear on each pin (few thousandths of an inch) will result in quite bit of chain "stretch" when multiplied by 134 links. Engine timing chains should be replaced every 100,000 miles. The chain tensioner are robust and will usually last the life of the engine. Automatically replacing it is always safest, most expensive and usually not needed. Inspection of the cam sprocket, tensioner and rails may indicate some of these parts are needed. Good experienced eyes and good decisions will help you from replacing parts which are good. The original aluminum/rubber chain rails do not break off and fall in the chain after years of use. Mercedes changed to plastic replacement chain rails in later years. The plastic rails become brittle after years of use and exposure to engine heat and chemicals. A loose chain banging on these brittle plastic chain rails can break them off and wipe out the timing chain and cam. Upper chain rails can be replaced if needed without major difficulties.
Bad tensioners are rare, but can cause clattering during cold starts even after a new chain has been installed. The tensioners are hydraulic and spring activated. Dirty, sludgy oil may cause them to malfunction.
