1993 Chevy truck loud lifter noise when the truck motor is warm.what is the cause?

If it is only doing it when it warms up then the problem is likely to be that the oil is too thin. A 93 probably has some miles on it so switch to a high mileage motor oil and what I like to do is use a 20% mix of lucas oil stabilizer. If you use 5 quarts of oil in a change just use 1 qt oil stab and 4 qts oil and that should firm things up under the hood.

When the oil thins out after it is heated it loses viscosity and will also flow out of the hydraulic lifters too quickly. I have been using the above mentioned mixture in my diesel engines that have 9000 hours on them ( about 500,000 miles ) and they are not in need of rebuilding any time soon, the stabilizer also extends your oil change time but I never do that.

Oil is cheaper than a rebuild so change it often.