What is the likely reason for the sound my Subaru makes when switching gears?

I drive a 1999 Subaru forester (yes it's very old, but it's only gone 165000 KM) with an automatic transmission and it has recently started making a kind of vibration/grinding noise when it reaches around 2000 rpm and switches from second to third gear. The noise is kind of a WRRRRRR(?) noise that then stops as soon as the car has switched gears. A couple of times recently the check engine light has also appeared, but then disappeared again, typically within the first 5-10 minutes of driving, after which it does not show up again. From googling a bit I have deduced that one problem could be the Transmission fluid needing to be switched. This is my first car and my car know-how is minimal.