I think you are getting the same noise I have. I thought it might have been a lifter since I'm running .644 lift on a 228 degree intake lobe and high spring pressure, but that checked out.. I thought it might have been a loose flywheel bolt that maybe didn't get Lock-tite when I had the new SPEC clutch put in.. but with the clutch in and idling, it doesn't change. The noise goes away when the rpm's go up with mine, so it's not bearing related, and likely not lifter related. Oil pressure is 60psi at cruise. I am 100% sure mine is an exhaust leak. I think the rear header bolt drivers side got stripped, and not to mention the collectors were slip-ons, that were attemted to be welded, but I'm betting a weld came loose with all the vibrating the engine does.
I've had the noise for almost 2 years now. I hear it in the same place, mostly under the car. I did notice the other day that there were some black marks coming from the passenger O2 bung on the Y pipe, so I really need to get that checked out if it's leaking there.