Doofus

My car has been making a whining noise for a few weeks now and I finally got it down to a garage to listen to.

Upshot seems to be it's a bearing in the transfer box. Can someone explain to mechanically what's going on? Plain terms are best

I also need a few questions answered (don't want much do I )

He said normally the 'whine' is very noticeable but in my case realistically he only heard it when I pointed it out. It was almost a suggestion that I could just leave it as is

I assume the problem will worsen over time. The question is how long? (or is this a 'piece of string' question?). Does it need fixed immediately or could I wait until the noise gets more noticable?

He is getting back to me this morning with a quote for work, the parts are going to be too bad he said but it's the labour that'll be bad. Any ideas how much a job like this SHOULD cost ? .. it's a main dealer we're talking here

Really any information or help would be appreciated.

Many thanks in advance,
Doofus.