*Copied from Popular Mechanics (good, basic reference for stuff like this)...
Whenever loud ticking is involved, the most likely source is the valvetrain, but the reason for that ticking isnt always clear. Most cars use hydraulic valve lifters that rely on oil pressure to maintain a tight clearance between the camshaft and rocker arm. If the pressure isnt right, there is play between the lifters and the rest of the valvetrain, which causes a noticeable tap. Its possible that cold oil at startup may help pump up the lifters, but at normal temperatures the viscosity drops, and may be too thin to correctly work the lifters. As the oil breaks down after a few thousand miles it thickens, which eliminates the noise.
Mechanical issues may also be the cause. Double-check that youre using the correct filter, and if you're not, consider switching to the manufacturer-supplied part. The flow rate through a generic filter could be incorrect, creating insufficient engine oil pressure. The worst-case scenario is that your oil pump is slowly dying and you need a replacement. I hate to say it, but driving the vehicle this way for 20,000 miles hasnt done the engine any favors. Get thee to a mechanic as soon as possible for proper diagnosis and remediation.
