Been practicing noting on and off for a couple months. I find myself having occassional doubts about whether I'm practicing effectively. I'm confused about when the noting of one sensation ends and another begins. For example, when noting the breath, I cannot always feel individual breathing sensations of rising and falling but more so one elongated sensation. And when I do notice a sort of separation in the breath, I think I may just be feeling the flutter of my heartbeat. Should I note the breath while I can feel it or only when a sensation ends and another begins? How does one develop the perceiving of vibrations or frequencies of sensations? 
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Just note what you perceive. That is the point - to look objectively at reality, at what is really there, at what you experience, rather than seeing what you expect. You don't have to understand it. Things will become clearer in time you will perceive greater and greater subtleties. Don't be attached to expectations, just observe the present moment as it unfolds.

You are more likely to experience vibrations if you as sit still as possible for longer / more numerous sessions being as relaxed as possible. (Don't take that as a recommendation that you sit in uncomfortable postures without moving and injure your knees or spine or anything else.) When I went on retreats we would do 5-10 minutes of walking meditation between 40 minute sessions of meditation. 

Thank u, very helpful!

I feel like this is the classic "expectation, vs. Realty" thing IMO. Because this exact thing happens to me. When you read a lot of theory, and meditation instructions, you build up in your head what it will be like. But once you actually sit there, and begin, you experience reality directly. What actually happens is the medition. The fact you actually see the breath as a smooth long breath IS the object. That's what's happening now. You can note it however you want, you can note whatever part of it you want. And you can note as fast or as slow as is helpful. And you can expand to include other mind categories of objects to note, such as the rest of the body, feelings, images, thoughts, craving/aversion/tuning out. Things like dullness or clarity. And also core sense objects close to the head, like perception, attention, intention, space. This paragraph is described more eloquently and helpful by Shargrol, for I am a mere noob c:  https://shargrolpostscompilation.blogspot.com/p/blog-page.html?m=1 Look up noticing versus noting. There is a big difference. You're gonna wanna look up the Shooting Aliens analogy, which describes noting. And also the Kazoo Player analogy, that explains what happens when you eventually switch from noting, to purely just noticing. Both are in MCTB

Good luck friend!

I don't know if this makes sense outside my head, just some ideas for you to tinker with.