La cisterna del baño hace ruido y tarda mucho en llenarse.

No. Here la cisterna means the toilet tank / cistern, the part that fills with water after flushing.

In Spain, la cisterna del baño is a natural way to refer to the toilet tank in the bathroom. It does not mean the whole toilet.

Related words:

• el váter = the toilet
• el inodoro = the toilet
• la cisterna = the tank/cistern part

Because de + el contracts to del in Spanish.

So:

• de el baño ❌
• del baño ✅

This is a standard rule:

• de + el = del
• a + el = al

Example:

• la puerta del baño = the bathroom door
• voy al baño = I’m going to the bathroom

Because hacer ruido is a very common Spanish expression meaning to make noise or to be noisy.

So:

• La cisterna hace ruido = The tank makes noise / is noisy

You could also say:

• La cisterna es ruidosa = The tank is noisy

But hace ruido often sounds more natural when talking about a machine, pipe, appliance, or mechanism that is producing a sound.

This structure is very common in Spanish:

• tardar en + infinitive = to take a long time to do something

So:

• tarda en llenarse = it takes a long time to fill up
• tarda mucho en llenarse = it takes a very long time to fill up

Breakdown:

• tarda = it takes time
• mucho = a lot / a long time
• en = to
• llenarse = to fill up

More examples:

• Tardo mucho en vestirme. = I take a long time to get dressed.
• El ordenador tarda en arrancar. = The computer takes time to start up.

Because llenarse means to fill up / to become full, while llenar usually means to fill something.

Compare:

• Lleno la botella. = I fill the bottle.
• La botella se llena. = The bottle fills up.
• La botella tarda en llenarse. = The bottle takes a long time to fill up.

In your sentence, the tank is not filling something else; it is becoming full itself, so llenarse is the natural choice.

Here se is part of the pronominal verb llenarse, which means to fill up or to become full.

It does not mean itself in a literal English sense. English often uses an intransitive verb here:

• The tank fills up
• La cisterna se llena

After tardar en, the verb stays in the infinitive:

• tarda en llenarse

So the full idea is:

• it takes a long time to fill up

Because mucho modifies the verb tarda, while muy modifies adjectives and adverbs.

Here:

• tarda mucho = it takes a long time

Compare:

• mucho with verbs:
  
  • come mucho = he eats a lot
  • tarda mucho = it takes a long time
• muy with adjectives/adverbs:
  
  • muy lento = very slow
  • muy rápido = very fast

So:

• tarda muy ❌
• tarda mucho ✅

Yes, that is also correct. The order can change without changing the core meaning.

• La cisterna del baño hace ruido y tarda mucho en llenarse.
• La cisterna del baño tarda mucho en llenarse y hace ruido.

Both are grammatical. The difference is mainly about emphasis:

• first version emphasizes the noise first
• second version emphasizes the slow filling first

In Spain, baño usually means bathroom. In everyday use, it can also refer to the room where the toilet is.

So:

• la cisterna del baño = the toilet tank in the bathroom

Depending on context, people might also say:

• la cisterna del váter
• la cisterna del inodoro

But del baño is completely natural if the location is clear.

It is fully understandable across the Spanish-speaking world, but cisterna is especially natural in Spain for the toilet tank.

In other regions, people may also use other words, such as:

• el tanque del inodoro
• el depósito
• la caja del baño in some local varieties

Also, in Spain:

• váter is very common for toilet

So yes, the sentence fits Spanish from Spain well.

Yes. Cisterna is a feminine noun, so it takes:

• la cisterna
• una cisterna

That is why the sentence starts with La cisterna...

Yes, depending on context. Hace ruido can mean:

• makes noise
• is noisy
• is making a noise

The Spanish present tense is often broader than the English present simple. So in context:

• La cisterna hace ruido could mean it generally makes noise
• or right now it is making noise

If you want to stress that it is happening right now, Spanish could also say:

• está haciendo ruido

But in everyday speech, hace ruido is often enough.

Because once the subject is clear, Spanish usually does not repeat it unnecessarily.

So:

• La cisterna del baño hace ruido y tarda mucho en llenarse.

After y, it is understood that tarda still refers to la cisterna.

Repeating it would be grammatical but less natural:

• La cisterna del baño hace ruido y la cisterna tarda mucho en llenarse.

Spanish generally prefers to avoid that repetition when the subject is obvious.