Rover 75 heater not always comes on

My car heater (blower) doesn't always come on when I start the engine. Most times it won't come on, but every now and again it starts blowing. When it doesn't it sounds like it wants to because you hear what sounds like sand going through the tubes inside the car. I have asked at a garage for an estimate but was told could be quiet expensive to fix. Can anyone give me some clues to what is happening so I can sound a little more educated to the problem. I hate to be taken for a fool and pay more when the car isn't all that valuable but it's all I have.