Could a shower make a whistle sound like it answered me?

That room can make sounds hard to place. Tile and glass reflect them, while running water masks quieter noises, so a whistle from outside the bathroom or another part of the home might seem closer than it is. The timing would be startling, but by itself it doesn't tell you what caused it.

I'd start with ordinary checks: turn off the water, listen from the hallway, and see whether anyone else heard it or whether it happens again. A single odd sound isn't enough to conclude the house is haunted. If it repeats, note when it happens and look for a consistent source before assuming anything unusual.