have to; are allowed to

[Lenka]

Which one do you consider right?

(in a letter from a girl spending her holiday in Japan to her friend)

...
And you have to / are allowed to make a loud noise when you drink tea! (In fact, it's polite if you do!)
...

[Coffa]

Which one do you consider right?

(in a letter from a girl spending her holiday in Japan to her friend)

...
And you have to / are allowed to make a loud noise when you drink tea! (In fact, it's polite if you do!)
...

The second sentence is intended to make a contrast. Therefore, the first sentence should not say '...have to make a loud noise...', because it implies an imperative. You must indicate that making a loud noise is permissible (something that is perhaps odd to Western ears), but not mandatory. So, '...are allowed to...' is the correct form.

Actually, I would write (in this context): "...and you can make a loud noise when you drink tea! In fact, it's polite if you do!"

Please note that this is a commentary on a Western IMPRESSION of the Japanese Tea Ceremony, not what a Japanese native would consider proper, or indeed mandatory, in such circumstances.