I haven't noticed this in 3 months but today when I started it up it sounded like the muffler was very noisy. Once I drove it a bit, the issue went away.

I have had this issue 2 or 3 times before but like I said this is the first time again in at least 2-3 months.
I called the dealer they told me it probably wasn't the exhaust.

What could this be? The weird thing is it goes away after driving for 3-5 minutes.

2017 Lancer ES 2.0 auto FWD

 

Seen very similar issues on CJ lancer
It gets quieter after a few minutes as it warms up
It was a small leak in the exhaust, it was the doughnut gasket on the downpipe.
A leak test can be done at idle to prove it.
Google it
Usually takes 2 people to test.
Hope this helps

The reason it got quieter after a few minutes as the heat from the exhaust caused the exhaust pipe to expand and reduce the gaps