its hard to imagine a hydro lifter bleeding down during the cycle of a 4 stroke motor, it just doesnt seem physically possible with their tolerances. Even the loose ones would take more time than a 10th of a second! Im thinking they are not pumping UP. The way a hydro lifter works is like a jack: oil under pressure is admitted through the the seep hole on the side past a check ball/disk into a small reservoir under a piston that is sealed by the OD of the piston and the ID of the lifter body, and in the same operation, the pressurized iol is also routing up the lifter to the seat hole where it feeds up the hollow pushrod and oils the rocker and its associated lubrication path. When this reservoir is full, the check valve seats and your basically running a solid lifter due to the fact that oil cannot be compressed and it has nowhere to go unless it seeps past the check valve (possible but unlikely in volume) or past the tolerance (~.0005!) of the piston (even more unlikely unless you can see an obvious issue just looking at them) you do have sufficient oil pressure in the lifter galley and the lifters are not stuffed with bearing glitter? Your rollers may be worn on the shafts or the noise is in the valve train. Do you have shims between the rockers to valvetrain specs? There can be side to side motion on the shaft if the rockers are not shimmed. Id pull a lifter and simply pump it with a pushrod submerged in oil. Does it pump up? Even the loosest race lifters will bleed down in ~10 seconds, not tenths of a second. Johnson lifters has a pdf paper on this but I cant seem to find it now.