Nissan Repair: Engine Sounds, variable valve timing, cam followers

Question
How much will it cost for the heads to be rebuilt? It seems like the clicking sound is actually coming from the back cover instead of the front cover, but I will take a screw driver to figure it out, I think my friend actually has a stethescope.

Thanks
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Followup To
Question -
I could do the work myself with some direction I might be able to get from a friend. The oil was changed regularly, what are all the parts I would need to replace? How can I avoid this in the future? Also, how dangerous is it to drive?
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Followup To
Question -
I'm having alot of unwanted noise coming from my engine, which someone had said might be a "misfire" I think is what they said. Could you take a look at this video: http://chr1shill.ath.cx/97maximaengine.wmv

It's a video of my engine with the "clicking sound" coming from where the video is shot.
Answer -
Chris,

It sounds like the variable valve timing mechanism is defective.  If the oil was changed regular you would be out about $1500 bucks to get it fixed at a shop.  If you can do the work yourself it is about half that.
Answer -
Chris,

First off get a screwdriver and make sure the noise is coming from the valve cover and if it is you need to remove the valve covers and look at the cam followers and see which ones are loose.  The advance and retard mechanism on the cams is probably stripped out.  So, if that is the case take the heads off and take them to a shop for a rebuild.  Then, replace the heads with new gaskets and such and you are back in business.  But, remember to check the valve cover with a screwdriver to be sure that the noise is coming from there.  It is a known problem with these engines and you will hear it by placing your ear on the handle of the long screwdriver when you put the other end on the valvecover.

Answer
Chris,

Now that sounds good and the heads might not need to come off to do the repair.  Take the rear valve cover off and see what is making the clicking noise.  YOu probably have a plugged up cam follower dampner.  Basically, you wiggle each rocker and find the ones that are loose.  They normally do not have slack in them.  If one does it is loose and needs to be adjusted or replaced.  Let me know what you find.  Also, on the missing problem have you changed the plugs and how are the coils?  You see, it could be as simple as one cylinder not getting spark and that could throw the engine off for the idle and stumble problem.