Long exposures are noisy

The pictures themselves are great - but the camera (Pentax K100D) makes
an absolute racket.

There's a bit of noise during the exposure itself, but afterwards, while
I presume the camera is processing it, it sings and coughs to itself at
length. Is this normal?

Also, it seems to take the camera as long to process a 30-second
exposure as it does to take the picture itself (i.e. before the camera
is ready for further action). Why should this be the case?

Daniele