Translation of “to talk behind someone's back”

Question

What is the typical Spanish translation of the English idiom "to talk behind someone's back" (as in saying something bad about another person to others instead of to them directly)?

Answer 1

One typical translation is

Hablar a espaldas (de alguien).

The DRAE also suggests:

Hablar por las espaldas.

1. loc. verb. Decir contra alguien, en su ausencia, lo que no se le diría cara a cara.

Answer 2

The most common are:

Hablar a sus espaldas

Hablar a espaldas de alguien

Also, there's the phrase:

Le deben estar pitando los oidos

It's used when someone talks behind the back of someone else, to indicate the degree of the talking (that what they are saying is really bad).