Lifter Ticking?

I am at my wits end trying to figure out how to fix the lifter noise in my son's '99 Grand Am with a 3.4. I am positive that it is lifter noise and not piston slap. The car has had the LIM gasket replaced twice. Once by a GM dealer and once by us. The car has 106K miles on it. The lifter noise started last fall. At that time we tried running some Risoline through the motor and that didn't help. Last month we tore into the motor and replaced all of the lifters, making sure to put the push rods back exactly where they came from. A visual inspection was made of the cam surface and it showed no usual wear on the noisy cylinder. All lifter bores were cleaned and checked for blockage. After re-assembling the motor we still had the exact same noise, so we replaced the push rod, and then the rocker arm. We still have the exact same noise that we had when we started. We have run a compression check on all of the cylinders and have found them all to be with in 10% of each other. We have not run a leak down test yet. It is the No. 4 cylinder Intake Valve, that is making the noise (front middle). The lifter appears to be pumping the same amount of oil through the push rod as the quiet ones. We ran the motor with the valve cover off to verify this.

We are ready to take another look at the motor, but I'm not sure where to go from here. If anybody could shed some light on how to solve this I would be very thankful.