I've noticed a loud humming noise coming from car only when I'm travelling between 48-55 mph. Fine up to 48 and once faster than 55 it's fine. Very strange I thought. On investigation online wheel balance or racking may be problem but no effect on steering. Any ideas?????

1. Possible saw-toothing of rear tyres. If you have anything other that directional tyres swap the rears side to side and that may cure it. If you have directionals, you could try swapping front to back on the same side.
2. Teach the car some tunes that you like!  

Alpaca syndrome..... 

 

First check tyre pressures and tyre condition.