No hi ha tant soroll al carrer com ahir.

Because haver-hi is the Catalan pattern used to mean there is / there are.

• hi ha = there is / there are
• no hi ha = there isn’t / there aren’t

So:

• Hi ha soroll = There is noise
• No hi ha soroll = There is no noise

The little word hi is part of this fixed expression. You normally do not drop it here.

In hi ha, hi does not translate neatly word-for-word in normal English. It is part of the existential structure meaning there is / there are.

Historically and grammatically, hi is a locative-type pronoun, but in everyday learning it is best to treat hi ha as a chunk:

• hi ha = there is / there are
• no hi ha = there is not / there are not

So in this sentence, don’t try to translate hi separately. Learn haver-hi as one unit.

Because tant is the form used before a noun.

Compare:

• tant soroll = so much noise
• tan fort = so loud

A helpful distinction:

• tan
  • adjective/adverb
    
    • tan alt = so tall
    • tan ràpid = so quickly
• tant / tanta / tants / tantes
  • noun
    
    • tant soroll = so much noise
    • tanta gent = so many people

Since soroll is a noun, tant is correct.

Tant ... com is the Catalan structure for comparisons of equality with nouns, often meaning as much/many ... as.

So:

• tant soroll com ahir = as much noise as yesterday

In a negative sentence like this one, the overall meaning becomes:

• No hi ha tant soroll al carrer com ahir
  = There isn’t as much noise in the street as yesterday

So tant ... com works like English as much ... as.

Here soroll is being used as an uncountable noun, like noise in English.

That is why Catalan uses tant rather than a plural form:

• tant soroll = so much noise

You would not normally say tants sorolls in this context unless you meant different separate noises in a more specific or unusual way.

Because a + el contracts to al in Catalan.

So:

• a + el carrer → al carrer

This is very common:

• al cotxe = to the car / in the car depending on context
• al parc = to the park / in the park

In your sentence, al carrer means in the street.

Ahir means yesterday, and placing it at the end is completely natural in Catalan.

The sentence is comparing the amount of noise now with yesterday:

• No hi ha tant soroll al carrer com ahir
• literally: There isn’t as much noise in the street as yesterday

Catalan often puts time expressions like ahir at the end, though other placements can also be possible depending on emphasis.

Not with the same meaning.

• No hi ha tant soroll ... com ahir = There isn’t as much noise ... as yesterday
• Hi ha més soroll ... que ahir = There is more noise ... than yesterday
• Hi ha menys soroll ... que ahir = There is less noise ... than yesterday

Your sentence uses a negative equality comparison:

• not as much ... as

That is different from a direct more than / less than comparison.

Because Catalan often leaves out the article with uncountable nouns when speaking generally, just like English often does.

So:

• hi ha soroll = there is noise
• no hi ha tant soroll = there isn’t so much noise

If you added an article, it would sound more specific and would usually change the nuance.

A learner-friendly approximation is:

no ee AH tan soo-ROYL al ka-RER kom a-EER

A few useful notes:

• hi ha often sounds smoothly connected in speech.
• soroll has the ll sound typical of Catalan; depending on accent, it may sound somewhat like ly or y to an English speaker.
• carrer has a trilled or tapped r sound.
• ahir usually has the stress on the last syllable.

Pronunciation varies a bit by dialect, but that approximation is a good start.

The structure is:

• No hi ha = there isn’t / there aren’t
• tant soroll = as much noise
• al carrer = in the street
• com ahir = as yesterday / as there was yesterday

So the sentence builds like this:

No hi ha + tant soroll + al carrer + com ahir

This is a very normal Catalan order. English uses a slightly different structure, but the Catalan sentence is straightforward once you recognize:

• no hi ha = existential negative
• tant ... com = comparison pattern