Here is a great explanation from Ike (WDW MAKER) on SVTPeformance.com. "The sound is simply a worn valve guide allowing the valve stem to move. This is caused by overheating of the area due to poor coolant flow in the part of the head. The lack of bronze valve guides in our heads makes them even more susceptible. The Jan05 heads come with revised coolant passages keep the exhaust valve guides cooler. This may also help to keep #7 and #8 cylinders running a bit cooler.
Will running a car with the tick result in some sort of catastrophic failure? Highly doubtful. It's mostly an annoying sound that you have to live with. I'm sure the worn valve guide causes a bit of inefficiency due to blow-by around the seals. I doubt it's enough to notice until it has been run like that for quite a while. I personally wouldn't and didn't want to live with it. Others chose to leave well enough alone and have yet to see any significant issues."
LDC sells a cooling mod that improves coolant flow, which in turns helps to prevent this issue from occurring.
This problem sounds like a light ticking coming from the driver’s side valve cover, and is most noticeable when the car is warm. If you can hear the noise inside the car while idling, you most likely have the problem, otherwise, the ticking you hear is most likely injector noise (normal). The only permanent fix for this issue is installing the latest factory heads, released in January, 2005. The part numbers are: Left side head Part # 2C5Z-6049-BAB Right side head Part # 2C5Z-6049-CAB
As of 11/10/06 the discount cost of these heads (complete with cams and valve train) is $1,358.00 plus core charges, from FORDSVTPARTS (Steve) on SVTPerformance.com. Steve can be reached by PM on that website, or by E-mail FORDSVTPARTS@HOTMAIL.COM. You can also call him at 1-800-328-9552.
Drivers head $616.00 plus a $150.00 core charge Passenger head $742.00 plus a $150.00 core charge
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RE: light ticking sound - 
