Noisy Cam followers ( Hydraulic Tappets) on an Alfa 24V Engine
This information pertains to mitsubishi V6 engines, but is reported as working on the Alfa V6 As Well.
On the V6's there is a device in the valve train called a valve lash adjuster (VLA). This device is like a little piston and cylinder filled with oil. It is different from a hydraulic lifter, and functions to keep the slack in the valve train to a minimum as the engine ages. The VLA has a little ball valve held in place by a spring, and uses oil pressure to take in oil and place constant pressure on its valve. Dirty oil from even several oil changes ago (or a previous owner) can leave particles in the ball valve that hinder its ability to remain pressurized. This leaves slack in the system and you hear it as clicking. It is also caused by getting air bubbles past the check valve that is now stuck at the top of the little cylinder, unable to leave via the ball valve at the bottom.
SYMPTOMS: A valve train clatter on startup that does not go away when the engine is warmed. If it goes away, this is not your problem.
Low oil - now or previously.
Dirty oil - now or previously.
High oil - now or previously (con rods slap the too high oil and "foam" it).
Parking on a steep hill - now or previously. With the engine off, this encourages the VLA to drain its oil (running steep hills are not a problem, unless it's so steep the oil is getting "foamed" by the con rods).
Parking for long periods - now or previously. Over time they'll drain.
Now that I've made you all paranoid, here's the laughably simple fix. The VLA's were designed to respond to a pattern of gentle revving to flush the oil out of the little cylinders (those clever Mitsubishi engineers, eh?)
1. Warm up the engine.
2. With the engine in neutral, gently rev it from idle to 3000rpm over a period of exactly 15 seconds. Your goal is a constant gradual increase to 3000rpm. At the 15 second mark, drop the throttle (foot off the gas) to idle. Now let it idle for 15 seconds to complete one 30 second cycle
3. Repeat the 30 second cycle in #2 from 10 to 30 times. If the problem persists after 30 cycles, this is not your problem. It fixed mine after only 5 cycles - total silence.