Some dogs are more than just occasional growlers. They growl when something's wrong but also when leaves fly by, shadows on the wall move or the imaginary mouse in the garage is back. Dogs growl for many different reasons, but it's important that you address -- and correct -- the behavior as soon as possible to avoid larger problems in the future.
Say "no" as soon as the growling starts. Just a single, firm "no!" should get the dog's attention if not get him to calm down. If the growling is about an object -- a toy, a bone, a bowl of food -- immediately take it away. Wait for Doggie to calm down, command him to sit, then return the item. If he growls again, repeat the "no" command and take away the object again. After a few times, he should understand that growling results in his losing something he values.
Feed Fido only after giving a command. One of the most common reasons dogs growl is over food. So ask him to sit down before you give him the bowl. Or, feed him from your hand to help him learn it's okay for people to be in close proximity to him while he eats. If he still growl, stand right next to him as he eats. Unless you're afraid of getting bit, try touching him -- just slightly -- on the back while he eats. This should reinforce the idea that it's okay to have you around. After all, your intentions are not to steal his food.
Distract him. If he's growling about something you have to do that he doesn't like -- such as clean his ears or treat a cut -- move his focus elsewhere. Is he all growl and no bite? Then bring a friend to help you feed him treats, or wave a toy in his face while you're doing what needs to be done.