Nobody can tell you specifically where it is. Based on an accurate description of the knock your hearing, an experienced mechanic--one whose repaired a number of knocking engines--can give you an educated guess of what might be loose; example: "rod bearings typically are "double knocks, increasing in frequency and intensity as the engine reves, especially when warmed up", or "single, lower frequency knocks could be main bearings", etc. Bottom line, a knocking rod bearing will have slop in the bearing, just like a knocking main bearing. Use plstigauge to check clearance on each bearing. Of course, you've already checked all accessories to insure there OK, right? Also, at idle, I've heard a number of "knocks" caused by exhaust pipes hitting something; often sounds like a rod bearing, but normally disappears when the engine is reved. A better description of what your hearing, including temperature of engine, oil pressure, affect of reving the engine on the knock will allow a better guess. Also, probably good idea to run a compression check.