Whirring sound when decelerating

More then likely this a gear or a bearing in the drivetrain. When you accelerate you are taking the slack out of the gears so it stops making the noise.

Here is a guide that will help with this. You will want to focus on the differential noise in this guide.

https://www.2carpros.com/articles/whirring-sound

Let's run through this and go from there. Thanks

I was told when I got my oil changed last week that I should get my differential fluid changed as well. Would this solve the problem? Or is it a bigger issue?

I can't tell you if it will solve the problem or not because if the damage is already done then the new fluid will not stop the noise. However, I would definitely do it because it can't hurt.

Here is the process from the manual.