Il bollitore si spegne da solo, ma il microonde fa ancora troppo rumore.

Because spegnere and spegnersi are used differently.

• spegnere = to turn off something
  
  • Spengo il bollitore = I turn off the kettle
• spegnersi = to turn off / go off by itself
  
  • Il bollitore si spegne = The kettle turns off

So the si shows that the action happens to the subject itself. With appliances, this often means it switches itself off or it turns off automatically.

Here da solo means by itself, on its own, or in natural English, often automatically.

So:

• Il bollitore si spegne da solo = The kettle turns itself off by itself / on its own

It does not mean the kettle is alone in an emotional sense. It just means no one has to switch it off.

Italian normally uses the definite article very regularly with nouns like il bollitore and il microonde.

In this sentence, the speaker is talking about specific, known appliances, so il is expected:

• il bollitore = the kettle
• il microonde = the microwave

Leaving out the article here would sound unnatural in standard Italian.

Yes, in everyday Italian il microonde is treated as a singular noun meaning the microwave oven.

It is basically a shortened form of forno a microonde. Because microonde originally comes from a phrase containing a plural-looking word, it can seem strange at first, but in common usage:

• il microonde = singular
• i microonde = plural

Many speakers also avoid the issue by saying il forno a microonde.

Because fare rumore is a very common Italian expression meaning to make noise.

So:

• Il microonde fa rumore = The microwave makes noise
• Il microonde fa troppo rumore = The microwave makes too much noise

Italian often prefers this structure where English might also say is noisy.

Yes, you could. Both are natural, but they are slightly different in focus.

• fa ancora troppo rumore = focuses on the noise it produces
• è ancora troppo rumoroso = describes the appliance as noisy

So both work, but fare rumore sounds very idiomatic and concrete in this context.

Here ancora means still.

So:

• fa ancora troppo rumore = still makes too much noise

Be careful, because ancora can also mean again in other contexts. You understand which meaning it has from the sentence.

Because ancora is modifying the whole verbal idea: still makes too much noise.

The most natural order here is:

• fa ancora troppo rumore

That places ancora close to the verb and gives the neutral meaning still. Other word orders are possible in Italian, but this is the most standard and natural one in this sentence.

Because troppo can be used with either a noun or an adjective, but the structure changes.

• troppo rumore = too much noise
  
  • noun phrase
• troppo rumoroso = too noisy
  
  • adjective phrase

In your sentence, rumore is a noun, so troppo rumore is correct.

The present tense is used here to describe a general fact, usual behavior, or normal characteristic.

So the sentence means something like:

• The kettle switches itself off, but the microwave still makes too much noise

It is not necessarily talking about only one single moment. It can describe how these appliances normally behave.

Yes. That would also be correct.

• si spegne da solo = more everyday, literally turns off by itself
• si spegne automaticamente = more explicit, a bit more technical or formal

In many situations they mean almost the same thing, but da solo sounds more conversational.

Because the sentence joins two complete clauses:

• Il bollitore si spegne da solo
• ma il microonde fa ancora troppo rumore

In Italian, a comma before ma is very common when two full ideas are being contrasted. It works much like English punctuation in a sentence such as The kettle turns itself off, but the microwave still makes too much noise.