I have an issue with my Macbook fan whirling loudly.

If I bring it in to the Apple store to get it looked at and they determine something is wrong with my fan, will they fix it on the spot or will I have to leave it there for a couple of days? I still have warranty.

In my experience, you will have to leave it. Technicians have a queue of work orders so they don't idle around.

I recently needed a battery replacement on my macbook pro. I made an appt at square one (cause i could get next day), arrived at my appt, they diagnosed my problem, took my computer and asked me to come back in 2 hours. I shopped for 2 hours and returned to pick up my computer and was on my merry little way.

Edit: i think it will depend a lot on if they have the part in stock. they told me I was getting the last macbook pro 13 in battery they had in stock and if they hadn't had one still I would have needed to leave my computer and get it later in the week.