1. Listening at doors includes like activity at other portals such as
windows. It is accomplished by moving silently to the door and
pressing an ear against it to detect sound.
 
 

Hearing Noise is simply listening intently.

The thief and his or her
accomplices must themselves be quiet (but not silent as in moving). This
function can be repeated as often as desired. It requires a full minute to
listen, i.e. one-tenth of a normal turn, or time equal to a melee round.
Note that sleeping creatures, undead, and many other creatures do not
make sounds discernible through a portal. Success informs the hearer that
someone or something awaits beyond the portal. (DMG)




Q: Is the thief's hear noise ability
continuous, or does it require concentration?
Is the ability strictly
limited to listening at doorways?

A: The hear noise ability is neither automatic
nor continuous. The thief must
quietly concentrate on hearing noise; if he
is wearing a helmet, he must take it off. A
thief is not limited to listening at doors; he
can hear faint sounds from any source.
(142.10)

DMG:

Hearing Noise:
This is pretty straightforward.
The thief, just as any other character,
must take off helmet or other obstructing headgear in order to press his or her ear to the door surface in order to hear beyond.

see Listening at Doors +
see Hearing Noise +