I have taken some photos of the doser valve, these have been taken with the top engine cover off, as you can see the doser valve it has two large air duct pipes running downwards out of it, when mine developed the problem the left hand pipe coming out of the base of doser valve as in picture would suck in flat for a second or to having switched off engine as the groaning or farting noise happened.
The next picture shows to the right of doser valve there is just room to push your hand down & feel a small connecting pipe maybe a 1/4" diameter this is the one you can pull off to see if it changes the noise as it happens, you cannot see this pipe from this position but it is down there, you can also follow the pipe as it runs across the engine with a black braded plastic cover this pipe runs between the doser valve & the solenoid.
The next picture shows top back left of engine where the solenoid is mounted just out of view at the rear top left of engine, this is held on with two small nuts & you can follow the same black pipe from doser valve to it but please note you cannot see this part but you can feel for it obviously making it hard to change this is where the noise comes from on mine, it maybe the only problem you have which would be the solenoid it is worth getting this changed first the price I've been given is £52.00 & half an hour labour to fit, hope this helps.