[Three_thirty_I]

Mine did something like this once briefly some years back, but in my case, was driving as normal, and when I stopped at the lights, all of a sudden the idle went very low which immediately prompted me to blip the throttle, it settled and then the lights when green so had to drive off. Car was fine after that. But I have had other niggling issues that could be related, and I am leaning more and more to suspecting my Vanos solenoids might just be tired. I've also cleaned and swapped mine around numerous times, in the past I've always just soaked them in some petrol, but earlier this year I decided to blast them with some carb cleaner which has a nifty nozzle, so literally blasted the cleaner in all the access holes and then let it stand some more in some that I had put in a small thin container. The car still has the odd hesitation or low rpm stutter, but have to say, seems a bit better, which makes me more confident that it's a Vanos solenoids issue. Also, since yours is the same sort of age, are cars both have the older part number Vanos solenoids which were apparently known to give problems, these were later replaced with a new part number replacement.

But you say your idle jumps around quite a bit, mine has had this random thing where it will be idling, and then all of a sudden the revs pick up to around 1500 rpm for a second or so, then returns to normal idle by itself. But then the throttle feels weird and laggy, and I even had it where the engine will not run past 6000 rpm on WOT when this happens. So I'm also torn into considering the Eccentric Shaft sensor - years ago, a BMW specialist experienced this same anomaly while testing after cleaning my injectors, and a code came up saying Valvetronic Faulty or Slow Plausible (or something along those lines), so he replaced the Valvetronic motor and ultimately this didn't really solve anything, and to add insult to injury, when I collected the car and drove it home, it was doing the same thing! So it could be the sensor, and yet, generally, the engine performs strongly, it's just those brief moments when things feel odd. And, what makes me suspect the Vanos more than anything else, is I typically experience this stumble issue when cruising in a taller gear, then quickly applying throttle to accelerate or overtake a car, that's when I get that annoying stumble or flat spot, if I back off and try it again straight after, then it doesn't do it.

The rattling noise that you are hearing when the idle is super low when releasing the clutch is most likely the dual mass flywheel that is chattering a lot, so probably not a good idea to leave it doing this unnecessarily...