El ruido era horrible.

Because era is the imperfect form of ser, and it describes an ongoing background situation in the past.

In El ruido era horrible, the idea is that the noise was horrible over a period of time, or as a general description of the situation.

If you said fue horrible, that would sound more like:

• it was horrible as a completed event
• you are viewing it as a whole, finished fact

A useful contrast is:

• El ruido era horrible = the noise was horrible / the noise kept being horrible
• El ruido fue horrible = the noise was horrible in that event, seen as completed

So era is very natural for description.

Spanish often uses the definite article more than English does.

Here, el ruido means the noise, referring to a specific noise in the situation being talked about. Even when English might say just noise in some contexts, Spanish often prefers el ruido.

For example:

• El ruido era horrible = the noise was horrible
• Había mucho ruido = there was a lot of noise

So the article is not unusual here at all.

Ruido is usually treated as an uncountable or mass noun, like noise in English.

So Spanish commonly says:

• mucho ruido = a lot of noise
• El ruido era horrible = the noise was horrible

You can use the plural ruidos, but that usually means different noises or individual sounds:

• Oía ruidos extraños = I heard strange noises

In this sentence, ruido refers to noise in general, so the singular is the normal choice.

Some Spanish adjectives change form for gender, and some do not.

For example:

• bonito / bonita
• alto / alta

But horrible is an adjective ending in -e, and many adjectives of that type stay the same for both masculine and feminine nouns:

• el ruido horrible
• la situación horrible

It does still change for plural:

• el ruido horrible
• los ruidos horribles

So horrible agrees in number here, but not with a different masculine/feminine form.

In Spanish, adjectives often come after the noun, especially when they are simply describing it in a straightforward way.

So:

• el ruido horrible = the horrible noise

In your sentence, though, the adjective is not directly next to the noun because it comes after the verb:

• El ruido era horrible

That structure is very common:

• noun + ser
  • adjective

It works just like:

• La comida era buena
• La película era interesante

Spanish can sometimes put adjectives before nouns, but that often changes the tone or emphasis. The basic, neutral pattern is usually noun first.

Both ser and estar can mean to be, but they are used differently.

Here, ser is used because horrible describes a quality or characteristic of the noise in that situation.

• El ruido era horrible = the noise was horrible

With estar, the sentence would sound much less natural, because horrible is not usually treated as a temporary physical state of the noise in the way estar often works.

In general:

• ser = identification, description, characteristic
• estar = state, condition, location

So era from ser is the expected choice here.

Normally, no. That would sound unnatural in standard Spanish.

Horrible in this sentence is describing what the noise was like, so Spanish uses ser:

• El ruido era horrible

Estar is more natural with adjectives that describe a state or condition:

• La puerta estaba abierta
• Yo estaba cansado

So for this sentence, era is the correct and natural option.

In Spain, ruido is pronounced approximately RWEE-dho.

A few details:

• rui- begins with a kind of glide, close to rwee
• d between vowels in Spain is often softer than an English d
• the stress is on RUI: RUI-do

So:

• ruido → RWEE-dho

The r at the beginning is a single tapped/flapped r, not a long English r sound.

Because both words already follow normal Spanish stress rules, so they do not need written accent marks.

• era ends in a vowel, so the stress naturally falls on the second-to-last syllable: E-ra
• horrible ends in a vowel, so the stress naturally falls on the second-to-last syllable: ho-RRI-ble

Spanish only adds a written accent when a word breaks the normal stress pattern or for certain other spelling distinctions.

Yes. Terrible is also very natural here.

Both work:

• El ruido era horrible
• El ruido era terrible

The difference is mostly one of nuance and personal choice. Horrible can sound a bit stronger or more vivid, but both are common and correct.

Yes, but it is less neutral.

The normal, most straightforward order is:

• El ruido era horrible

If you say:

• Era horrible el ruido

that can sound more literary, emphatic, or marked. It might be used to highlight horrible first, as if you are stressing how awful it was.

So it is possible, but the original sentence is the basic word order learners should use first.