Why would a 1995 GMC 350 make a tapping sound when cold that gets louder when warm?

If it is a Tapping sound it is normally a lifter. Could also be a cam, pushrod, and or a rocker arm. Tapping is normally upper motor. If it is a knocking noise it is normally in the lower motor. If the noise gets louder when warm most likely a lifter is going bad. As the oil heats it thins slightly causing more oil to bypass inside the lifter which it is not supposed to do