struggles to get into gear and makes a loud whistling sound.

Hi and thanks for using 2CarPros.com. First, we need to determine if the transmission fluid is clean and full. If the fluid is clean and full, I recommend checking for trouble codes. Most parts stores will do it for free. However, here is a video showing how it is done:

https://youtu.be/YV3TRZwer8k

If there is an internal transmission issue or a sensor, solenoid it should set the check engine light. When you retrieve the code, let me know what it is.

This is just the start. Get back to me with any information you can find.