Ninja - do you have lane departure warning on your car?
I noticed that my vibrations felt alot like the LDWS system going off, but just a little less intense. I decided if I can't do anything about the source of the vibrations, perhaps I could do something about the part it is transmitted to; the steering wheel. Not a fix, but a band aid. At this point I had nothing to lose.
I took off the main s/wheel bolt and put the airbag back in place and started the car. With the wheel loose, i felt much less vibrations, so I removed the wheel completely and cut some 6mm foam numberplate stickers into a ring. I stuck the ring at the location of the red circle here to act as a spacer/damper between the wheel and the hub it sits on (this is not my s/wheel but it illustrates the location):
The stuff I used looked like this after cutting it:
This helped reduce the vibrations by about 30%, which was noticeable, but they were still there.
So next I decided to look at the LDWS motor:
The vibrating body sits on those little black rods, and it's free to move back and forth along those rods - that's how it vibrates when the system is activated; it moves back and forth very quickly. I felt how easily the body moves back and forth by just pressing it with my finger and I figured that perhaps the vibrations that are coming into my s/wheel are being amplified by this. It is after all designed to make the steering wheel vibrate so it's not impossible to think that if the car's natural vibrations are enough to get this thing moving, then I would feel that as increased vibrations.
Not sure either way but it took 3 minutes to remove it and after I did, vibrations reduced to around 20% of what it used to be! A serious improvement - I'm very used to diesels here in the UK as most of our cars are, and now what I'm left with is just 'normal' diesel idle vibrations. I can keep my hands at 3 and 9 o'clock at traffic lights and there is no irritating buzz, just very mild typical engine idling type of sensation.
I can live without LDWS while I have the car, and can refit the motor in 5 minutes when I come to sell it. The airbag on these cars come off soooooooooo much easier than e46 air bags.
On a separate note, I haven't set up my ISTA yet (having some problems getting the cable to recognise the car) but I did clean my MAF with electrical contact cleaner. I must say the car feels more spritely and a little smoother at lower revs such as moving off from a stop. That was a good shout on your part. When I get my ISTA working I will reset the adaptations on the MAF also.