Wish I saw this thread sooner could have saved you some time. I cant believe they were trying to tell you its a rod bearing! This noise is common on E90s, F25s and a few other models. Its just noise transferred through the transmission, the plug sometimes fixes it. I have heard them MUCH louder. The fix to this is to "clock" the TQ converter. Meaning unbolt it from the flywheel and rotate it, rebolt it to the flywheel. This fixes it. FYI this only affects Auto cars, not manuals.
2013 Alpine White F10 535i. JB4 S2, Quad Tip Magnaflow Exhaust, H&R Springs
2011 335ix Le Mans Blue w/ M Sport SOLD
2003 Turbo Cobra 765rwhp/650rwtq 9.77@126 pump gas, lifting @ the 1000ft mark