What is citta in the four frames of reference? I am trying to understand the four frames of reference. I could understand the body. I could comprehend the sensations (to be honest not completely). I could not understand the mind (citta). 

This has been my observation. I'm sensitive to noises. I feel irritation (sensation) and anger generated towards the person who created it, and the mind stays angry and continues to in that way. Is this what citta is ? That which continues to be in that state even after the sensation that caused it ceased to exist ?
Citta is mind. In Four Frames of Reference, it means your overall state of mind:


  "And how does a monk remain focused on the mind in &amp; of itself? There is the case where a monk, when the mind has passion, discerns that the mind has passion. When the mind is without passion, he discerns that the mind is without passion. When the mind has aversion, he discerns that the mind has aversion. When the mind is without aversion, he discerns that the mind is without aversion. When the mind has delusion, he discerns that the mind has delusion. When the mind is without delusion, he discerns that the mind is without delusion.


If you feel a strong emotion in your body immediately after the noise - that is vedana (what you called "sensation"). The state of anger generated towards the person is a kind of aversion.

But this is not complete awareness of citta yet. In order to see the entire citta you need to expand your awareness further.

In this case, the reason you feel irritated when the noises happen, is because in your mind (in your citta) you have established a certain intent, a certain desire, or a certain expectation, -- an expectation of a quiet environment conducive to concentration. This expectation serves as a background or counterpart to noise, it is what the noise is compared with - and since the noise and this expectation are obviously in conflict - you get irritated and angry, because of the mismatch. Now, this expectation of quiet environment probably comes from an even deeper intent, for example, you may be trying to achieve a certain state that you have read about in a book. Etc.

At every moment of time we have a certain frame of reference inside of which we are operating. This frame of reference is made from some concepts. Most of the time, we are not actively thinking about these concepts but they stay "on the periphery" of our minds. These peripheral framing concepts are maintained or renewed through a mechanism known as "upadana" in Buddhism. The way it works is, every time we hear, see, or think about something connected with these concepts, we "feed" them, so they keep staying on our (background) mind.

Now this is a more complete picture: you have read about some state in a book, now you keep thinking about that state, you wanted to attain a similar state, you think that in order to attain this state you need to concentrate on something, and therefore you see all noises as enemies, getting in the way of you attaining your state. Therefore you get irritated and angry. (Your situation may be different, I'm making this stuff up, but you get the idea. You need to learn to clearly see your "background intent" - in every case, not just in this case.) On top of this, you may have some preconceptions about noisy people, that identify them with being sloppy, uncultured, uncaring etc.

So your complete state of mind at that point includes all of that - from very high-level mental frameworks - to very specific thoughts and sensations.

This entire configuration, when you take all of it at once and look at it "panoramically" has a certain flavor or a certain taste. It can't always be described in words, but it is something you can experience if you look at all of the above all at once.

That panoramic assessment of the all-inclusive quality of your mindstate is what we're after in this meditation. This panoramic awareness of the entirety of one's mind, including its implicit frames-of-reference, is called cittanupassana or simply vipassana.