This is my guess but I think the definition of PT is broad, if your T goes with your heart beat or blood flow then it is PT. I don't know if tonal PT is rare but I haven't heard from someone else having it.
Classic PT is caused by turbulent blood flow near the ear, or blood flow that is too close to the ear, usually low frequency. There is also Ménière's disease. The type of PT that some us have isn't blood flow at all, but rather sensory nerves that are activating the auditory system, and since nerves are kept energized by blood flow, we hear a higher pitched hissing that goes with our heart beat.
Totally guessing, but in your case I would guess that whatever is causing your T is fluctuating with your heart beat because it is energized by blood flow. For example, if your cochlea is causing the T then maybe it boosts the T each time it receives a new supply of energy (oxygen). I don't know your age, health, etc. but maybe improving circulation would even out the fluctuations, if your circulation isn't perfect.
In my case if I'm sitting down and my heart is resting, then I get up and walk up some stairs, my PT pounds louder for a few heart beats as my heart is beating harder. At times I can hear exactly what my heart is doing.
Have you noticed anything like this?