That image seems to indicate that its not the pituitary.
Pineal gland, more like it:

At any rate its in a certain context that "pineal pituitary dynamic" is referenced, think amrita and the sphenoid sinus but that's by no means doing it justice. The thing is, if you're looking for a "location" then you're doin' it wrong

Its...an energy dynamic. Same with the lower dantien, you are given the rough trailmap, but you have to really go walk it to experience it. 2 inches under the navel, front:back 3:7 only tells you a location, but what you're doing isnt traveling somewhere to see a sight, its expressing an energy dynamic, for lack of a better term. Niwan is under baihui, so draw straight lines from baihui and yintang and that is the location.
Breathing with the nostrils is a waste of time for meditation. (imho

) If you think of a hose, what happens to the walls of the hose when pressure is added? They firm up. It is the same with your air passageways - that's what led me to come up with an air passage identification exercise. Spending a little time seeing how and where you may effect the local air pressure with all of the various parts where air physically goes to. Then, from a standpoint of action, derive inaction - let go of them completely and do not use the sinuses or any of it to buffer the incoming air pressure. It takes emphasis off of the diaphragm (and by extension, the "fundamental 3" structures of breath mechanics, diaphragm-psoas-perineum.) Also, the vagus nerve passes along portions of it, providing feedback. What do you want that feedback to be? ;)
That actually helps the breath disappear, so like a car engine's piston you have one surface performing the chamber's volume adjustment (compression; in the car it reaches criticality and sparked, in human air simply moves in and out.) Reduces the 40cps excitatory firing of the olfactory cranial nerve.
So really it becomes managing the interplay of the diaphragm-psoas-perineum dymanic while holding the awareness at the niwan. (I like saying niwan because it is not referring to the specific physical structure. We're talking energy dynamics here

) Anything not necessary, eliminate it. Work until you cant hear yourself breathe. Then put in earplugs and repeat. Dunk your head in the bath if you need to, heh. On the way figure out how to let the heart sit comfortably and shine happily and the breath will be even finer.
Set program, repeat ad infinitum. That provides inertia and leverage. That sets the stage for 'real work' realizing the nature of awareness and such

Regarding a pulse that gets loud in your head, eliminate the tension. You're not forcing anything. If anything it is more like upgrading bandwith...or "relaxing to increase flux density." Correlating ohm's law, tension restricts. Make sure your brow is not furrowed, a slight smile helps open baihui, relax all of the muscles of the face and eyes. So if you are trying to locate the niwan and do it too forcefully and get a headache or such a pulse whereas it is a distraction or uncomfortable, bring the energy down. You dont want to leave energy to get stuck in the head, that is why it is important to relax. When you've had enough, dont keep going in a session. Where exactly you bring it down to isnt as important, you can bring it to the heart if that works, or the solar plexus, or the lower dantien. The yongquan on the feet, even. There's also a good exercise called pulling down the heavens that would help for that also. But I want to stress that it is important to proceed gently with the niwan - that's why I've tried to give analogies like setting a lamp on a pedestal in the middle of a room - because the lamp doesnt struggle to put out the light, it simply shines from its location. That is all the awareness is doing, so this should not be an intense thing at all. Light and comfy

When done, use an open palm to rub the baihui, rub along the ub & gb meridians and out the shoulders, also go across the forehead and down the face, and settle to the lower dantien before entering stillness a la hexagram 2.
Whenever you do energy practices, anything working with an energy center, always enter deep stillness afterward. Plan that as part of your meditation time, it is every bit as important as the active portion of the energy work. There's no point in doing an hour of some energy center work if you're not going to let that resonance settle deeply into your being after having done the work. Far better to do 10, 20, 30 minutes of the active stuff and 30, 60 minutes of utter stillness afterward. This is where the active portion of ingraining an aware breath protocol manifests a great, efficient breath when you've let go of everything to go into deep stillness - it makes it that much deeper. But not having that, no problem, correct the thought and habit and establish the habit-energy. And make the times something reasonable for whatever your "current state" is, dont expect too much of progress, let go of any outcomes anyway. That's not to say one cant push himself, but more that one will have to know himself and be wise to pitfalls that may inhibit or divert practice.
Now, once one is familiar and comfortable with "fixing the spirit at the seat of awareness" that can be done any and even all times, I think that is one of the vinayana precepts for monks, "not letting awareness leak away"...but I wanted to reiterate some of the fundamentals of these processes so they are done safely and wisely.
The tao reflects upon itself, smiling