Ich drehe mich im Flur um, weil ich ein Geräusch höre.

Because the verb here is sich umdrehen, which means to turn around.

German often uses separable verbs: a verb with a prefix that moves to the end of the clause in a normal main sentence.

• infinitive: sich umdrehen
• main clause: Ich drehe mich um

So:

• ich drehe = I turn / I rotate
• ich drehe mich um = I turn around

The um belongs to the verb and is not a separate word with its own meaning here.

Because sich umdrehen is a reflexive verb. That means the action is done to the same person who is doing it.

So:

• Ich drehe mich um = I turn myself around / more naturally, I turn around

The reflexive pronouns are:

• ich → mich
• du → dich
• er/sie/es → sich
• wir → uns
• ihr → euch
• sie/Sie → sich

In this sentence, the subject is ich, so the reflexive pronoun is mich.

Because sich umdrehen takes a reflexive pronoun in the accusative, not the dative.

So the correct form is:

• Ich drehe mich um.

not:

• Ich drehe mir um.

Some German reflexive verbs use accusative, some use dative, and some can use both in different structures. This one normally uses accusative.

Because umdrehen is a separable verb, and in a normal main clause, the prefix goes to the end.

So:

• infinitive: sich umdrehen
• conjugated main clause: Ich drehe mich um.

This is very common in German:

• anrufen → Ich rufe dich an.
• aufstehen → Ich stehe auf.
• mitkommen → Kommst du mit?

The same thing is happening with umdrehen.

Because weil introduces a subordinate clause, and in German subordinate clauses, the conjugated verb usually goes to the end.

So:

• main clause: Ich höre ein Geräusch.
• subordinate clause with weil: ..., weil ich ein Geräusch höre.

This is one of the most important German word-order patterns to learn.

Compare:

• Ich bleibe zu Hause, weil ich krank bin.
• Sie geht weg, weil sie keine Zeit hat.

In all of these, the verb is last in the weil clause.

Im Flur means in the hallway or in the corridor.

It is a contraction of:

• in dem Flur → im Flur

This is very common in German:

• im Haus = in dem Haus
• im Zimmer = in dem Zimmer

Here, Flur is a masculine noun:

• der Flur = the hallway

Because this sentence describes a location, not movement into a place.

German uses different cases after in:

• dative for location: where?
• accusative for direction/movement: where to?

Here, the speaker is already in the hallway:

• Ich drehe mich im Flur um.
  = I turn around in the hallway.

If there were movement into the hallway, you would use accusative:

• Ich gehe in den Flur.
  = I go into the hallway.

So im Flur is correct because it answers where?

Ein Geräusch is in the accusative case because it is the direct object of höre.

The verb hören usually takes an accusative object:

• Ich höre Musik.
• Sie hört ein Geräusch.

Since Geräusch is a neuter noun (das Geräusch), the nominative and accusative forms with ein look the same:

• nominative: ein Geräusch
• accusative: ein Geräusch

So the form does not change visibly here, but grammatically it is accusative.

German present tense often covers both simple present and present progressive meanings from English.

So:

• Ich drehe mich um can mean
  I turn around or I am turning around

And:

• ich höre ein Geräusch can mean
  I hear a noise or I am hearing a noise, depending on context

German usually does not need a special progressive form like English am turning.

Yes, but the grammar changes.

You could say:

• Ich drehe mich im Flur um, weil ich ein Geräusch höre.
• Ich drehe mich im Flur um, denn ich höre ein Geräusch.

Both can mean because, but:

• weil introduces a subordinate clause, so the verb goes to the end:
  weil ich ein Geräusch höre
• denn is a coordinating conjunction, so normal word order stays:
  denn ich höre ein Geräusch

So both are possible, but weil and denn do not behave the same grammatically.

Yes. German word order is flexible in some ways, as long as the verb rules are followed.

For example, you could also say:

• Im Flur drehe ich mich um, weil ich ein Geräusch höre.

This puts more emphasis on im Flur.

But the key rules still stay the same:

• in the main clause, the conjugated verb is in second position: drehe
• the separable prefix goes to the end of that clause: um
• in the weil clause, the verb goes to the end: höre

So word order can shift for emphasis, but the basic grammar pattern remains.