Hydraulic Valve Lifter Adjustment
Um, cold or hot, it doesn't much matter for hydraulic lifters. See section 60-10 of the Opel Factory Service Manual. Set # 1 piston at TDC, firing position. Loosen valve train pivot nut until rocker arm is loose, tighten to eliminate looseness, then another full turn tighter. Repeat on other valve. Rotate to next firing order TDC. 1,3,4,2. Some prefer 1/2 or 3/4 of a turn, but the effect is the same.
All you are doing is compressing the internal spring a bit by depressing the lifter plunger. The spring then supplies a minimum "force" on the valve when they are closed, and the oil in the lifter transmits cam lift (and hence force) to the valve to open it.
If the lifter continues to tap, the valve may be sticking (not fully closing). Or the lifter is worn out, and won't hold oil pressure.
HTH