I shall describe a 2 recent sits.

Very quickly develop throbbing and fine vibrations and heat over forehead and behind eyes on starting observation of falling and rising of belly. Gradually the abdomen and breathing become very subtle and can tell that it is made up of small packets of physical effort rather than a smooth in and out. Keep noting whatever arises though the forehead sensations remain mostly the same (maybe become a little less prominent). Gradually notice a sense of diffuse softness all over the body (not vibrations as in the forehead) but almost like there was no solid form. Also notice that as awareness goes around the body it is associated with a sense of heat everywhere it rests.

Another sitting proceeds almost similarly to the one above but has another component. Suddenly notice a deep coldness that arises from the base of the spine and that ascends to the top of the head through the core. It becomes almost painful and is also associated with sweating. A little alarming (almost a little scary) but noting makes it go away in about 1 minute.

Questions:
1) What should be the approach to noting when I feel the diffuse soft humming all over? I am unable to discern any discrete sensations as they all blend into one soft nothing. Should I focus more on trying to discern something discrete within it all? Or should I ignore it and just focus on the breath? I have done both these approaches and don't see any difference in what happens next, which is mostly what I described already.
2) If in each sitting one is supposed to go through all the nanas one has cycled through before, how do I explain the experience I described in the second paragraph? I thought it may be the knowledge of fearfulness but I am not sure. Something similar but more painful had happened a few years ago but not anything since (though I have to admit my meditation has been irregular in this period).
3) Any other pointers are welcomed wholeheartedly.

Cheers!

To answer number 1, I suggest you take the sensation of "diffuse soft humming" as a whole, and do vipassana or whatever on that. One thing that has helped with my practice is this: there are various kind of focus: local/global, hard/relaxed, etc; if one kind of focus isn't moving things along, then try a different kind of focus.

With regards to the cool current, in yoga this is described as the result of kundalini rising up through the left channel (ida, I think). Then you get cold. You should be careful with this as if there is a serious unbalance you can become really really cold. If it goes through the right channel (pingala) it will feel really really hot. A teacher once recommended that in these extreme situations one should cover the body or take a cold shower, respectively. If kundalini goes through the center channel (sushuma), it will feel like bliss/sexual ecstasy (these three channels are all located inside the spine and are hard to distinguish).

I would suggest you try doing 5-10m of the microcosmic orbit before each sit (look up on the internet to see how), it should normalize things.

Thanks, Bruno.

Your comment on the kundalini is interesting. I shall look into the practice you suggest.