Hey, on the road atm so bare with me.

I'm hearing this very faint whistle sound inside when the car is off. Nothings running to my knowledge. I can't hear anything on the outside of the car. Does anyone have any clue?

Do you think it's a coolant leak? I have been needing to top up every 2-3 of months.

yes sounds like you have a small coolant leak under pressure it leaks

you need to do a pressure test