The procedure of changings cams is typically down without changings lifters,
   unless there was a pre existing problem with the lifters. I have seen this
   procedure down, and know of many others who simply received the cams and
   all was well. 

   There are a number of reasons why your lifters bled down and produced the
   racket you heard. If your mechanic did any combination of things he could have
   easily bled them down. Which is only a temporary problem, and usually will
   correct itself after running the bike long enough for the oil to warm up, and
   circulate back through the lifters. 

   In the process of installing the cams, the rockers base is removed, and the push
   rods are extracted, the lifters are also removed. Your mechanic should have bled
   the lifters before re-installing them. Putting all components back in the original
   position, maintains the same valve clearance, and an adjustment of the valves
   is not necessary. The critical part of assembling the engine to avoid lifter bleed
   down is bolting the rocker base back on. Because the Rocker base preloads the
   lifters, it is important to install each rocker base when the cylinder is at top dead
   center and the cam lobes are in the fully closed position. This way you are not
   forcing the rocker base down on lifters that are sitting on the high side of the
   cam. 

   One final note here. The design of the hydraulic lifters is a very basic simple
   design. They rarely fail, and unless they have dirt in them which blocks the check
   valve, they are not the problem.