That's due to a gate being used which I found offputting on her recordings years ago.

http://en.wikipedia.org/wiki/Noise_gate

There are many things going on with Ms. Krall's albums. For example, the instrumental/orchestral parts are recorded completely separately and the vocal is placed on top, close mic'd and with a reverb that has nothing to do with the sound of the instruments. It can make one a bit seasick.

While it is acoustic lite-jazz, her albums are substantially compressed - unlike even relatively recent past history where acoustic jazz was handled gently with open, transparent, clear sound. Compression can impact the sound in some odd ways.

Norah Jones' first album was also overly compressed - with little dynamics, impact, and interest (needing as much as it can get of each of these things, sorely lacking in all). Her second album was quite a bit worse - nasty sound.

I have no clue where all the noise comes from on Ms. Krall's albums. There is plenty of money, excellent equipment and good engineers available for her recordings - so why all the grainy noise?