I have a 1983 XJS V12 that I recently swapped another 83 motor into it with about 67k on the clock. I posted my noise sorrows recently on the XJS thread but thought I would get some input on this thread also. Basically what happened is I had fired it up a number of times and was working out the bugs when at idle it suddenly decided to start making a pronounced knocking noise. I feared I had dropped a valve or something, but I have ruled that out both because it is running so well now (I can balance a glass of wine on the intake at idle, no misfire at the exhaust) and the stethoscope test is showing me that the heads are relatively quiet and the noise is not emanating from the top of the motor. Oil pressure is very good and it is holding 20 psi when hot at idle. Like I described before, if it was a Ford or Chevy V8, I would think it is a rod/piston knock. The sound is much more pronounced when first started and quiets down considerably after it reaches temperature to where it is not noticeable standing a few feet away from the car. Before I installed the motor I resealed the top and bottom and checked a few mains and the bearings/crank looked great (obviously torqued back to spec/I did not check any rod bearings assuming the mains told the story). I guess my next step will be to crawl under the car and check the pan with the stethoscope. I would really hate to have to pull the engine again. Thoughts? I will tighten exhaust, manifolds etc. while I am down there and if I am lucky something is loose, but I am not holding my breath. Thanks.