Riding a motorcycle can be strenuous on many senses, including your hearing. Unless you've spent a great deal of money on a top of the line helmet, chances are you're probably hearing a lot of wind noise and if your bike is loud, you're hearing that too.
Wearing ear plugs can help 2 ways. First of all, it can protect your hearing. Long term riding with loud pipes or a lot of wind noise can damage hearing. Secondly, the right ear plugs can filter out un-wanted noise and actually help you hear the motor better. (This may not always be a good thing if you're paranoid about that slight knocking noise you can now hear)
A music store is a great place to get ear plugs. They have them with high/low filters that cost under 30 bucks and last for months. These ear plugs reduce most of the wind noise and much of the exhaust noise, but let you hear everything else.
If you spend a lot of time riding, you might consider getting a set made specifically for your ears, but be advised they are expensive; often over $100.00 a set.