Noticing Sensations

I was listening to a talk on YouTube about vipassana practice as taught by Goenka. The woman doing the talk said that the Buddha taught that every time a stimulus enters the sense door there is a sensation in the body and it is these sensations that we react to not the stimulus itself. I have tried the body scan technique quite a few times but not very successfully and so have ended practicing Mahasi Sayadaws technique instead which is focusing on the abdomen rising and falling. I have to say I'm not very aware of these sensations that are happening to every stimulus. Of course when I sit and meditate I notice some sensations such as itching, pulsing, pain etc but as far as the ones that are the result of every stimulus entering I'm not sure how I go about noticing those before I react. Also my understanding is that Buddhism also sees thought as a sense door so therefore thought also causes these sensations. Is that correct? In order to be a successful practitioner do I need to train myself to notice all these sensations? Is it good enough to just notice some like itching, trembling? Do I need to understand what causes them? Or is noticing them enough? Or do I need to notice the feeling tone associated? Or both?