Sorry I meant you CANT hear it when you stand next to the engine
An auto trans is supposed to have some issues with loose torque converter/flywheel bolts....I have read something about this somewhere.
This might be a broken cat....
hearing it more in the cabin tells me that its under the car, not under the hood so much.
see if you can jack it up or get it on a lift and have someone listen underneath....a broken cat might stop the rattle when the RPM goes up, then begin again as the rpm drops.. The flow of the exhaust pushes the pieces against the cat insides when rpm is up, but idle is not enough exhaust pressure to hold the chunks so they flop around in the cat housing and rattle like crazy... See about this asap because the cat will eventually clog and/or the chunks get blown into the mufflers and clog them.
That is, if you agree this is an irregular rattling type noise...fairly loud like a nickle in a beer can....tin.
If it rattles exactly in time with the engine, its not a cat.
if this is the click or ticking thats
very rythmic, but mild, try the MM. Just be sure to read Hibs post about other possibilities and mechanical issues that MM won;t cure.
This is
ALL about the definition of the sound..rythmic clicks are injectors, (everything gets progressively worse in this list) ticks or taps more likely to be lifters, raps are small end rod bearings, banging, big end rod...bumping/thumping/clunking ...spun main, when originating from the motor.
Then again,. I'm just guessing !:ugh
.....and yes, thats the correct MM engine repair in a bottle. Mystery oil for mystery noises.... (some folks hate hearing that ! ! )
Just ad a few oz to the motor oil, and a dose in the gas. You will notice it seeming to run better right away. Change the oil after a few days. Put easy miles on it with the MM in the oil. Freeway and normal driving is fine...just no street racing or high speed runs.
Give it 100+ miles or run that tank of gas out then change the oil.
MM thins the motor oil so there is less high-load protection. About the opposite of what STP does...
Hope this helps !
... in case it don't, find a mechanic in a shop when the mechanic in the bottle don;t fix it !