O câmbio e o escapamento fazem barulho.

Because the subject is plural: o câmbio e o escapamento = the transmission and the exhaust.

In Portuguese, when two singular nouns are joined by e (and), they usually form a plural subject, so the verb must also be plural:

• O câmbio faz barulho. = The transmission makes noise.
• O escapamento faz barulho. = The exhaust makes noise.
• O câmbio e o escapamento fazem barulho. = The transmission and the exhaust make noise.

So fazem is the 3rd person plural form of fazer.

Not necessarily. In Portuguese, fazer barulho is a very common expression meaning to make noise or to be noisy. The noun barulho often stays singular because it refers to noise in general, not to separate individual noises.

So:

• faz barulho = makes noise / is noisy
• fazem barulho = make noise / are noisy

You could sometimes hear plural ideas in other contexts, but in this sentence, singular barulho is the normal, natural choice.

Yes, very much so. Fazer barulho is a common everyday expression in Portuguese.

It can mean:

• to make noise
• to be noisy
• sometimes to cause a disturbance

Examples:

• As crianças fazem barulho. = The children make noise.
• Esse motor está fazendo barulho. = That engine is making noise.
• Não faça barulho. = Don’t make noise.

So in your sentence, it sounds very natural.

Portuguese often repeats the article before each noun in a list, especially when the nouns are separate, concrete things:

• o câmbio e o escapamento

This is more natural than leaving the second article out.

You may sometimes see article omission in headlines, labels, or very compact styles, but in normal speech and writing, repeating the article is standard and natural here.

Because that is simply their grammatical gender in Portuguese. Noun gender is something you usually have to learn together with the word.

Here:

• o câmbio = masculine
• o escapamento = masculine

That is why the article is o, not a.

A useful pattern is that many nouns ending in -o are masculine, though this is not a perfect rule.

In a car context, câmbio usually refers to the transmission or gearbox. In some situations, it can also refer more loosely to the gear shift system.

So depending on context, an English speaker might think of:

• transmission
• gearbox
• sometimes gear shift

In this sentence, transmission/gearbox is the safest understanding.

Escapamento usually refers to the car’s exhaust system, or sometimes more loosely the exhaust.

In everyday speech, people may use it for the exhaust area or exhaust pipe in general, even if technically the full system has multiple parts.

So for learning purposes:

• o escapamento = the exhaust

A simple pronunciation guide:

• câmbio → roughly KAHM-byoo
• escapamento → roughly es-kah-men-TOH

A few details:

• In câmbio, the stress is on câm.
• In escapamento, the stress is on men/to area, more precisely to: es-ca-pa-MEN-to? Wait—actually the natural stress is men? No: because it ends in -to, the default stress is on the second-to-last syllable: es-ca-pa-MEN-to. So the strong syllable is MEN.

More careful approximation:

• câmbio = KAHM-byu
• escapamento = es-kah-pah-MEN-too

In Brazilian Portuguese, the final o often sounds closer to oo than a strong English oh.

Yes. That is also correct, but it changes the feel slightly.

• O câmbio e o escapamento fazem barulho.
  More general or neutral: they make noise / they are noisy.

• O câmbio e o escapamento estão fazendo barulho.
  More immediate and specific: they are making noise right now.

So if you are describing a current problem with the car at this moment, estão fazendo barulho can sound especially natural.

Grammatically, people may understand it, but it sounds less natural in ordinary speech.

In normal Portuguese, with specific parts of a car being discussed, you would usually say:

• O câmbio e o escapamento fazem barulho.

Leaving out the articles can sound more like:

• a heading
• a technical note
• a list
• very telegraphic style

So for everyday conversation, keep the articles.

Because Portuguese usually says fazer barulho, not fazer um barulho, when speaking generally about making noise.

Compare:

• faz barulho = makes noise / is noisy
• faz um barulho estranho = makes a strange noise

So if you add um, it often sounds more specific:

• O câmbio faz um barulho estranho. = The transmission makes a strange noise.

But in your sentence, the more neutral and natural version is simply fazem barulho.

Usually, yes, or at least it suggests a noticeable sound that the speaker thinks is worth mentioning.

On its own, fazer barulho can simply mean to make noise, but in a car/mechanical context it often implies:

• unusual noise
• excessive noise
• possible mechanical trouble

So many listeners would understand this as a sign that the transmission and exhaust may need attention.

Yes, but the tone changes.

• barulho is more common and everyday.
• ruído is a bit more technical, formal, or neutral.

So:

• O câmbio e o escapamento fazem barulho.
  Very natural in everyday conversation.

• O câmbio e o escapamento fazem ruído.
  Understandable, but less colloquial.

In normal spoken Brazilian Portuguese, barulho is usually the better choice here.