What are all those sound notifications & how to get rid of them?

[Android Central Question]

I get unidentified sound notifications frequently on my Samsung Galaxy S5 phone. Verizon just tells me, "it's something updating".
What I really want to know is:
1. What caused the sound?
2. What apps cause a particular sound? The most common sound that I hear is the same as my email notification. I know, I know, I could change my email notification, but I really don't want to do that & I'd still have the unknown sounds.
a. Is there a way to determine which installed app (or what) causes a particular sound?
I located an app (PastNotifications) which I hoped would help, but it has added to the mystery. I continually see notifications of downloads that have already happened hours/days/weeks ago. For instance, sometimes I email a document attachment from my computer to myself and save it to the phone's sd drive. Or I send reminder email notes to myself, which I do not delete until it's no longer useful.
I confess that I'm terrible about keeping old emails that I should delete. Do they (& their attachments) continually download until deleted? If so, why do they notify me every time?

 

[B. Diddy]

Welcome to Android Central! Do they go away if you turn off NFC?