Previously had similar activity in Mac mail, but have all sounds off, so cannot say it is exactly the same.
Result of combination of using multiple devices (iPads, iPods, computers), multiple users sharing a mail account, and using a pop3 based mail account (yahoo). Pieces of mail get moved between folders on the web server when using other devices, then when you open Mail on Mac, you see a bunch of activity as it sorts the mail. When using Mail for a pop3 account, you see all the folders that exist on the web page down the left sidebar of mail. When you move mail to trash on other device, you will see activity on Mail next time you open it.
All activity (and visibility to online folders) stopped when I changed to imap.
I have started getting the 'whoosh' BEFORE the plane has even left the runway. The sound was always a confirmation that the message had been sent successfully, but now, the progress bar is still working away AFTER the whoosh sound has gone and, often, the message doesn't even go and is put in the outbox to attempt it again at another time. This is very unreliable information and I have come to distrust any of these 'confirmation' sounds. It works ok on my Mac - just the iPad which does it different.
Same thing here for nearly 3 years on my macbook air and Air 2013, both on 10.8.5. 20 years on Macs & have never had so many issues.
I will say that computer "crashes" are a thing of the past which is amazing, but so many other little issues arise constantly.
There has got to be a fix for this other than updating to a new OS which my 512GB Air does not have room for! a topic for another forum topic, but how on EARTH does a 512gb fill up on files considered "Other"??? never seen such a ting in all my 20+ years on Mac, and no APPLE utility to fix it, scan it, see what those files are.