tapping - late 2004 model
My Excursion has a tap in the engine that is sometimes very loud and at other times is barely noticeable. This has been going on for almost a year now without getting worse or better. The engine has 170k miles and has had head gaskets/studs, oil cooler, FICM and all drivers side injectors changed within the last 50k miles. The truck is stock, no tuner or upgrades. It runs great. Occasionally, I can feel a little shudder or miss at idle at a red light but only after idling for a minute or more. I've tried using a mechanics stethoscope to locate the sound but I can't find anything distinct. The loudest tap I can hear with the scope is touching the front of the heads - both sides about the same. I've checked the nuts on the flex plate, they are tight and in good shape. I've also changed viscosity of engine oil and tried several oil additives, but these didn't make any difference at all. The sound is similar to a lifter going out but I would think that the engine would miss if that were the case. I'm convinced that if this were a rod bearing, it would have had failure by now.
Anyone ran into this problem before? Any help or suggestions are appreciated. Thanks.