My two cents worth - if it were a shim problem - would not it cause a heavy tick instead of a solid knock? I suspect a bad hydraulic lifter that won't hold it's prime and has to be primed to build up the oil pressure every time the engine is started. After the engine has sat for a while, pull the valve covers one at a time (to prevent too much oil mess) and use an automotive stethoscope to pin point the bad lifter. Even just feeling the rocker arms should find the culprit when the lifter is knocking that hard. I had a bad hydraulic lifter on a VW Diesel that sounded like the bottom of the engine was pounding out - it was so loud. I pulled all lifters and sat them in oil for 30 minutes - all had strong resistance when compressed - but the bad one was like pushing on a sponge. Have your mechanic check for dirt in the offending lifter - or for the price - just replace it. THEN check the preload on all lifters. I could be wrong but for the preload to be that far off to cause a solid knock and then disappear as the lifter is primed after 30 seconds doesn't seem reasonable. The lifter should still tick after 30 seconds if the preload is all that is off. Just my opinion though.