Hello!

2006 Forester with a 4EAT automatic transmission. The fluid is fresh, the transmission runs perfectly with no complaints at all.
However, there’s a whining noise coming from the transmission that appears during coasting, but it feels like a "gear is engaged".
Other times, it feels like the car is in neutral and coasts quietly.
When the car is cold, it’s hard to hear the whine, but once warmed up — especially the hotter it is outside — the louder the whine gets.

What could be causing this whine in 1st–2nd gear while coasting at around 5–10 km/h?
In all other modes and speeds, the transmission is quiet.

Please advise.
Thank you.