The noise appears around 1 second after I switch off and disappears around 1 second after I switch on. This is why I'm thinking that the ECU detects something strange and switches to a degraded mode.
When I switch on, engine on or killed, the heat plugs are heating up (deduction from the sudden voltage decrease).
I can't say if there is an increase in consumption. If I had an iFCD (*) I don't think I would be playing with the engine
I have no idea what happens exactly. The noise is the same when I try to start the engine with the cable to the needle lift sensor disconnected. This may be another indication that the ECU falls into a degraded mode.
The noise is louder when the engine is pulling the car, than when the engine is idle.
The noise is present whatever the engine temperature. It may be louder when the engine is cold, but I can't say for sure as I switched off with cold engine while idling. Now I try to switch off only when the engine is idle, so the described method...
I'll look at the pipe tomorrow to see if some (strange) fume appears when switch is off.
Thanks,
Denis.
(*) the signal from the needle lift sensor may be my next game...