This video shows the procedure for the 3646 which I believe is very similar to the 3647. As for the fan try this page and this one.
We have a number of Dell 3647 SFF desktops at my office.  A couple of them have developed noisy fans, and I'd like to replace them. Watching a youtube video on cleaning a 3647 SFF, it appears the AVC fan does come off without removing the heatsink. Can anyone suggest where I can buy the fans only without the heatsinks that will plug and play?  Preferably new.  Thanks!
Inspiron 3647 SFF Fan replacement