Here's my 2 cents. Mechanics, being one myself can be a very expensive guessing game at the expense of the car owner. As BKS said could be as simple as tyres or as complicated as a FWD diff. Before you get any mechanic to rip into anything.. have a fiddle around yourself. In most cases when a bearing has failed you can feel play and/or vibration or hear a dry or failed bearing when u jack the car up and turn the wheel by hand. Do some research on this website and google for symptoms of failed components or causes of humming. Just to give you an idea of how broad the possibilities are, i had a similar problem on my sisters astra and it turned out to be air in the p/steer system. At low speeds no matter which way the wheels faced everything was normal, but as soon as you reach a speed of 90km/h the car would whine very loudly. Goodluck