Looking on the internet deeply has found these results:

on a side note is the most popular phrase on the web.

On the side note 17,970,000 results on the web	on a side note 116,900,000 results on the web More popular!
Some examples and use cases from the internet:	Some examples and use cases from the internet:
First, an interesting side note. Side note... maybe lock the door next time you leave for the night. I found a neighborhood dog up on the bar, drinking from the soda gun. The German Foreign Ministry indicated to me when I raised this in the debates that public acceptability would be strengthened enormously if everybody could have something familiar on one side of the note or the coin. Side note don't order the spinach. author, which support an increase in de minimis payments. On the positive side, I note the success of the amendments of which I am a co Side note: you no longer have a working microwave.	It doesn't look like this phrase is very popular! The search could not find any examples on the internet.

On the side note

17,970,000 results on the web

on a side note

116,900,000 results on the web

More popular!

First, an interesting side note.
Side note... maybe lock the door next time you leave for the night. I found a neighborhood dog up on the bar, drinking from the soda gun.
The German Foreign Ministry indicated to me when I raised this in the debates that public acceptability would be strengthened enormously if everybody could have something familiar on one side of the note or the coin.
Side note
don't order the spinach.
author, which support an increase in de minimis payments.
On the positive side, I note the success of the amendments of which I am a co
Side note: you no longer have a working microwave.

It doesn't look like this phrase is very popular! The search could not find any examples on the internet.