in addition to GTP Dad's suggestion, try putting some CD-2, Seafoam or Marvels Mystery oil in. If you let your oil changes slide a bit, or use Pennzoil, you may just have some sludge clogging the lifter up. The CD-2 fixed a friends van that was doing the same thing. It's cheap and easy and won't harm anything even if it doesn't fix it, it's worth a try. DO NOT use an engine flush (the type you put in just before changing your oil and let it run 5-10 minutes). Those chemicals actually break down the lubricity of the oil and can do MAJOR engine damage. The add-in detergents I listed above stay in and you let it run for the life of the oil change.