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[b]here i found a link to some information on it
its called Engine Tapping or Tickinghttp://www.jonko.com/forum/diagnosis/noise_engine_rattle.htm
Engine Tap or Tick
Engine tapping or ticking can be the result of a number of things. To accurately determine the cause of your problem, you should listen carefully to when the noise begins and ends, and how it varies (or doesn't) with the speed of the engine. Use the brief diagnostic steps below to find the probable source of your engine noise.
1. Check Your Gas
Make sure you are using the proper gas for your application. Check the owner's manual to find out if your vehicle requires a premium grade of gasoline and if so, use it. Low octane fuels combust more easily than high octane fuels and as such may result in "pre-ignition" or the pinging and rattling that you hear.
2. Check Your Situation
Many times a ping or rattle is the direct result of your engine running a bit hot or under stress. These situations can be caused by driving in high altitudes, uphill, through deserts, or in any othersituation where a load is placed on the engine. If the engine is running hotter than normal due to one of these conditions, the noise may be a direct result. Try using a higer grade fuel when you know you will be driving under stressful conditions in the future.
3. Overheating Elsewhere?
If your vehicle isn't under an unusual load, you may wish to check for other problems in the cooling system. Any issue in the system (from a faulty thermostat, to low fluid in the radiator) can cause the engine to run a bit hotter than normal and result in the pre-ignition ping.
4. Check the Timing
Older cars require that their timing be checked and adjusted periodically (check your manual for details.) If your car hasn't had the necessary work done, take it in to make sure the timing is correctly set. Newer vehicles are computerized and are timed by the onboard system so an issue with timing here will need a mechanic's oversight.
5. Is it Turbocharged?
If your vehicle has a turbocharger, a faulty knock sensor could be the problem. Turbocharged vehicles have a knock sensor that is responsible for keeping the engine from knocking and pinging. Have your mechanic test the sensor and replace if necessary.
6. EGR and More...
If none of the above remedy your situation we suggest taking a look at your EGR valve. If the valve is bad or sticking, or if a hose or wire leading to the EGR is disconnected, this may cause the pinging you hear.
here is another link:http://ca.autos.yahoo.com/maintain/engine_ticking_noise.html
The valves in your engine need to be adjusted.
There is a lot of sludge in your engine which is stopping oil from circulating properly.
The engine's valve lifters are collapsed.
One or more of the engine's valves are stuck.