agree with Trazor, shouldn't be a wheel bearing but depends on the mileage? (Nothing would surprise me with a Zafira though, they're ok but I'm fussy with my cars....) Anyway try the following out:
1. Does it get louder going round hard bends at decent speed? If so it is probably a front wheel bearing. (bend to left, weight goes to right so it would be the right wheel... etc)
2. does the whistling or whining change at different speeds? If it changes tone completely or sounds different at different speeds it could be somthing daft like a bit of trim, but a wheel bearing would be likely to stay at the same kind of low/medium drone.
3. Does it go away or change when you brake? If so maybe a bit of the braking system rubbing somehow. Jack up the car and try turning the wheels by hand and see if you hear anything, (or see anything), but you won't hear wheel bearings without the load of the car on the wheel driving (unless they're really knackered..)