I just press 'T' and the scan is automatic. No sound though(?)
...I had my discovery scanner on the same button as my primary fire. Whenever I was in a fight I kept hearing that sound and a message saying "No new astronomical objects detected" or something like that.
There is a difference between 'Scanning' as in analyzing an unexplored object you are targeting - this is when you target the object with T and keep it in your sights and should be taking something like 20-40 seconds depending on distance and plays a gradually increasing sound effect - and using the scanner to discover (mark new unexplored objects on your navigation)
The big boom is produced when you actually 'Fire' the Discovery Scanner - an equipped component