Super simple 2 minute way to add the sound of a motor to your bicycle.
When I was a kid we did this and had lots of fun. I just put these on my kids bikes and they thought it was awesome along with every other kid in the neighborhood begging me for balloons.

Step 1: Get a Small Balloon.

Get a small balloon and blow it up about two-thirds of the way. A water balloon size works well.

Step 2: Look for Place to Attach Balloon

Find a spot on the front forks or on the back to tie the balloon on to, that's not in the way of the brakes or the chain.

Step 3: Attach the Balloon.

Tie the balloon to the bike so that is sticking out and not touching the spokes.

Step 4: "starting the Motor"

As your riding your bike push the balloon into the spokes (instantly start smiling). alternatively you can push the balloon into the spokes before you start riding.

Step 5: Here Is a Video

Something is lost in the from in person to a cell phone video. But it sounds a lot better in person and while riding your bike a littler faster than I had the wheel spinning in the video. also you can put multiple balloons on at once and have several going at a time or start one after another when the "motor" backfires (pops). All I can really say is to try it and i'm sure you will have a blast. Then when your finished having all that fun please come back here and vote for me for the Bicycle contest.