OP: What is citta in the four frames of reference?
There 3 references to this. One from the Mahā Satipaṭṭḥāna Sutta and Satipaṭṭḥāna Sutta. Another called the 18 Mental Exploration found in Dhātu Vibhaṅga Sutta / Titth’ayatana Sutta / etc. And also the 3rd triad in doing Anapanasati found in Anapanasati Sutta / Mahā Rāhul’ovāda Sutta / etc.
And how, bhikshus, does a monk dwell observing the mind in the mind?
Here, bhikshus, a monk,
(1) understands a lustful mind as 'Lustful mind',
or, he understands a lust-free mind as 'Lust-free mind'.
(2) Or, he understands a hating mind as 'Hating mind',
or, he understands a hate-free mind as 'Hate-free mind'.
(3) Or, he understands a deluded mind as 'Deluded mind',
or, he understands an undeluded mind as 'Undeluded mind';
(4) Or, he understands a narrowed [constricted] mind as 'Narrowed mind'.
or, he understands a distracted mind as 'Distracted mind';
(5) Or, he understands a great [exalted] mind as 'Great mind',
or, he understands a small mind [unexalted mind] as 'Small mind [Unexalted mind]'.
(6) Or, he understands a surpassable mind as 'Surpassable mind',
or, he understands an unsurpassable mind as 'Unsurpassable mind'.
(7) Or, he understands a concentrated mind as 'Concentrated mind',
or, he understands an unconcentrated mind as 'Unconcentrated mind'.
(8) Or, he understands a liberated mind as 'Liberated mind',
or, he understands an unliberated mind as 'Unliberated mind'.
Mahā Satipaṭṭḥāna Sutta
There are 18 mental examinations:
one explores a form that gives rise to pleasure,
one explores a form that gives rise to pain [displeasure],
one explores a form that gives rise to equanimity.```
```(2) On hearing a sound with the ear,
one explores a sound that gives rise to pleasure,
one explores a sound that gives rise to pain [displeasure],
one explores a sound that gives rise to equanimity.```
```(3) On smelling a smell with the nose,
one explores a smell that gives rise to pleasure,
one explores a smell that gives rise to pain [displeasure],
one explores a smell that gives rise to equanimity.```
```(4) On tasting a taste with the tongue,
one explores a taste that gives rise to pleasure,
one explores a taste that gives rise to pain [displeasure],
one explores a taste that gives rise to equanimity.```
```(5) On feeling a touch with the body,
one explores a touch that gives rise to pleasure,
one explores a touch that gives rise to pain [displeasure],
one explores a touch that gives rise to equanimity.```
```(6) On cognizing a mind-object with the mind,
one explores a mind-object that gives rise to pleasure,
one explores a mind-object that gives rise to pain [displeasure],
one explores a mind-object that gives rise to equanimity.```
Thus there are six mental explorations with regards to pleasure; six mental explorations with regards to pain [displeasure]; six mental explorations with regards to equanimity.
‘The 18 kinds of mental explorations should be understood,’ thus it is said in this connection.


See: Titth’ayatana Sutta, Dhātu Vibhaṅga Sutta

Anapanasati Sutta
OP: 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 ?
When you hear the sound you get irritated, i.e., the latent tendency of aversion rising. You keep thinking about the sound and the person who made it keeps you angry. Continued thinking about the incident is mental proliferation.
“Bhikshu, as regards the source from which proliferation of conception and perception assails a person: if one were to find nothing there to delight in, nothing there to welcome, nothing to cling to—this is the end of
- the latent tendency of lust,
- the latent tendency of aversion,
- the latent tendency of views,
- the latent tendency of doubt,
- the latent tendency of conceit,
- the latent tendency of desire for existence, and
- the latent tendency of ignorance.
This is the ending of the taking up of the rod and the sword, quarrels, disputes, mayhem [strife], slandering and lying —here these evil unwholesome states cease without remainder.”
…
Avuso, dependent on the [eye | ear | nose | tongue | body | mind] and [form | sound | smell | taste | touch | mind-object], [eye | ear | nose | tongue | body | mind]-consciousness arises.
The meeting of the three is contact.
With contact as condition, there is feeling.
What one feels, one perceives.
What one perceives, one thinks about.
What one thinks about, one mentally proliferates.
From that as source, proliferation of conception and perception assails a person regarding past, future and present [forms | sounds | smells | tastes | touch | mind-objects] cognizable through the [eye | ear | nose | tongue | body | mind].
Madhu,piṇḍika Sutta
What needs to be done to quickly get rid of the anger is:
(2) Or, he understands a hating mind as 'Hating mind',
or, he understands a hate-free mind as 'Hate-free mind'.
Mahā Satipaṭṭḥāna Sutta
i. On seeing a form with the eye,
- one investigates the form that is the basis for mental joy,
- one investigates the form that is the basis of mental pain,
- one investigates the form that is the basis of equanimity.
ii. On hearing a sound with the ear,
- one investigates the sound that is the basis for mental joy,
- one investigates the sound that is the basis of mental pain,
- one investigates the sound that the basis of equanimity.
iii. On smelling a smell with the nose,
- one investigates the smell that is the basis for mental joy,
- one investigates the smell that is the basis of mental pain,
- one investigates the smell that is the basis of equanimity.
iv. On tasting a taste with the tongue,
- one investigates the taste that is the basis for mental joy,
- one investigates the taste that is the basis of mental pain,
- one investigates the taste that is the basis of equanimity.
v. On feeling a touch with the body,
- one investigates the touch that is the basis for mental joy,
- one investigates the touch that is the basis of mental pain,
- one investigates the touch that is the basis of equanimity.
vi. On cognizing a mind-object with the mind,
- one investigates the mind-object that the basis of mental joy,
- one investigates the mind-object that is the basis of mental pain,
- one investigates the mind-object that is the basis of equanimity.
Dhātu Vibhaṅga Sutta
- Know that the feels are impermanent so do not cling onto them
If he feels a pleasant feeling,
- he understands that it is impermanent;
- he understands that it is not to be clung to;
- he understands that there is no delight in it.
If he feels a painful feeling,
- he understands that it is impermanent;
- he understands that it is not to be clung to;
- he understands that there is no delight in it.
If he feels a neutral feeling,
- he understands that it is impermanent;
- he understands that it is not to be clung to;
- he understands that there is no delight in it.
If he feels a pleasant feeling, he feels it in a detached manner.
If he feels a painful feeling, he feels it in a detached manner.
If he feels a neutral feeling, he feels it in a detached manner.
Dhātu Vibhaṅga Sutta
Observe the mind
(C) The Third Tetrad: Observing the mind
(9) He trains himself thus: ‘I will breathe in experiencing the mind.’
He trains himself thus: ‘I will breathe out experiencing the mind.’
(10) He trains himself thus: ‘I will breathe in gladdening the mind.’
He trains himself thus: ‘I will breathe out gladdening the mind.’
(11) He trains himself thus: ‘I will breathe in concentrating the mind.’
He trains himself thus: ‘I will breathe out concentrating the mind.’
(12) He trains himself thus: ‘I will breathe in freeing the mind.’
He trains himself thus: ‘I will breathe out freeing the mind.’
See: Ānâpāna,sati Sutta, Mahā Rāhul’ovāda Sutta