When I put the handle down to vacuum, it makes a loud sound and doesn't suck up. But I don't see anything in the brush.

Sounds like the tube could be blocked.
Dyson's are easy to disassemble and trouble shoot.

You haven't mentioned which model you have, as the process varies on uprights as opposed to cylinder models.

Check out dyson's site as they offer a lot of helpful advice: http://www.dyson.co.uk/