My 2008 Honda CRV has a noticeable squealing noise coming from the engine compartment when the engine shifts down suddenly. An example would be when I am driving at highway speeds and a large incline begins and the car shifts down. The squealing noise may or may not occur but it also feels as though the car is loosing some power when this occurs as though a belt is slipping but it does not sound like a belt slipping. IT is much higher pitched.
When your transmission shifts down, your engine’s speed (RPM) suddenly increases. If the sound you hear is really a squeal, then it is an issue with your accessory belt. Either the belt is worn out or the tensioner is bad.
If this is happening with your AC on, try turning it off. If that stops or reduces the squeal, then this points to the belt. If it happens with the AC off, then try turning the AC on. If it happens sooner or louder then, that too points to the belt.