Forgive me .... but also posted on the "Dark Side" :)

My 1997 (98MY) 4.6 GEMS has always run like a Swiss watch. Last Friday, 38 degrees (100F) outside, go for a run, pull up and all of a sudden she has this rattle at idle. It is worse when in gear as opposed to in neutral. It really isn't a tick, more a clack and sounds for all the world like it is coming from the LH Cat.

I have had the screwdriver to the ear as can report there is definitely a solid "tick" coming from the cat, and can follow this up to the manifold, where it is also audible, maybe as loud, maybe not.

Now, it goes away over say 1000 RPM. It isn't always there. When it is, it is LOUD. Sometimes it might not be there at cold start, and then again it might not. Sometimes when hot and the oil is thin, but sometimes not. It sounds for all the world like a dodgy lifter, but using a stethoscope on the rocker covers, well they are quiet, whether the noise if present or not. I can't detect anything by putting the stethoscope on the head either.

So the noise .... it sounds mechanical, it seems to be coming from the LH cat. It is regular, like it is associated with a single cylinder, a regular clack, clack, clack as I said like a lifter.

So at the moment I'm lost.

I'm not sure it is mechanical.

Points for consideration. Temp is rock solid, mid gauge and always is, only ever drops on a long downhill run, water level never moves, on the mid seam mark of the expansion bottle hot or cold, no oil consumption, oil nice and clean, no blow by and no evidence of water contamination in the oil, i.e. the oil filler cap is dry and clean, 140k km 90k miles. Never had a noise of any sort until a couple of days ago.

Is it only when idling or is is worse when accelerating but once the engine is spinning faster you won't hear it? It could be the start of a head gasket blowing out the side which won't give you any coolant in the oil, overheating or any of the other 'normal' symptoms. If the liner is slapping up and down, it will be pressurising the cooling system. Could it be the cat has broken up and the bits are ratting?

Seems to be only at idle almost as though it was related to oil pressure. Then again today, it wasn't evident at all, no noise, sweet as a nut.

I'm sure something will show up at some stage, when it does, or more clues, I'll advise.

If it is the cat breaking up, you might find it can be dislodged to somewhere else in the exhaust with a run at higher revs, I had the same happen and it would consistently rattle at idle, a blast up the dual carriageway locked in 2nd stopped the rattle. Though its probably a sign of more to come if the cats started breaking up, and might dislodge more of it. Best solution would be to remove the cat and inspect it if possible, at least to see if anything comes out if you tip it on its end.