Jim,
My two cents.....if the sound you actually hear is the hollow rapping of a bad connecting rod then you should be able to recreate that sound at will my manipulating engine RPM.
Some of the later Vortec engines (like mine) did have issues and could make noise. The only way a rod would knock would be from excessive clearance or maybe lack of oil.
If it truly is a rod knocking it will probably get worse. There are other accessories that can mimic this sound strangely enough...AC compressors and alternators for example. I have heard of loose or cracked flexplates on auto trannys sounding this way too!
As they say, 'it will develop'..keep us posted and good luck.
Stuart
