An odd and baffling sound

[CorsePerVita]

So some time ago last year, my car developed a rough idle, feels like it's just rough on one cylinder, a slight miss if you will. I replaced the plugs and it went away (for a while). Mind you, it always had a hair bit of an odd clacking sound. It sounds a bit deeper, not like rod knock, softer, quieter, but a bit of a clack clack clack at idle. I am not the type to troubleshoot things in denial, so if I thought it was something severe like rod knock I'd have stopped driving it a long time ago. Thing is the sound never got worse, it always remained the same. Even today, never got any worse. The odd thing is that it will remain at engine temperature. However, once the thermostat opens and the oil starts moving through the cooler, the sound almost immediately dissipates and remains ever so barely faintly noticeable, and I mean hardly at all noticeable.

The odd thing is... I ONLY hear it in the cabin area. If I go under the car, nada. If I open the engine bay, nada. If I go to one of the wheel wells, nada. I do not hear it. So as I've been trying to narrow down WHERE it is coming from, I simply can't figure out where in the hell the sound is actually coming from.

At this point I've pretty much dismissed it as something loose that's vibrating, but it doesn't explain the slightly rough idle that smooths out once the engine gets at full temp and the thermostat opens.

It is quite baffling. Car runs fine, the idle is where it should be. The sound cannot be heard any other time other than idle, between 800-1200rpms, then it's non existent. It's rotational though, that's what is odd to me. It does coincide with the rpms of the engine.

Ideas? Suggestions on any good ways to rule out the BAD things and rule out the ODD things? If it's something serious I'd rather address it sooner than later. Since summer has arrived and I have multiple vehicles I can drive, it would not put me out of house and home to do a tear down IF it comes to that. (and how hard is it to do a basic rebuild? rings, bearings, etc)

Also... I AM due for a valve adjustment... so I'll be doing that this next week and I wonder if that could be related?

[tshebib]

I vote for a broken exhaust side head stud. Take the lower valve covers off and inspect/check each one.

[CorsePerVita]

Sounds like a good thing to check "While I'm In There" doing my valve adjustment. I didn't even think of that. Thanks tshebib. I'm open for any ideas and suggestions if anyone has other ideas but that's definitely a good suggestion.

[Daves911L]

It is possible you are describing normal 915 gearbox rattle (more like a clunking I always thought). I've never noticed it outside a car, but it can be very noticeable when you are sitting inside. It is temperature dependent since slight changes in RPM or smoothness of running have a big effect on those rotating parts inside the transmission and how they are greeting their neighbors.

[irl]

any update