Sound from my back tire when accelerating

Today I noticed that there was a sound coming from my right back wheel area that seemed to increase when I accelerated and slow down when I started braking. It was not present at all when I was not moving with the car on. I'm not sure how to describe the sound exactly, but it was kind of like metallic in nature. I stopped to inspect the car to see if there was something visible that I could notice, but there was nothing wrong that I could see.

I brought the car the few miles back to my house, once I stopped to look at it, and I figure I should take it to a mechanic to look at it, but I wanted to make sure that it doesn't sound like something for which it would be dangerous to drive it. Any advice on the matter would be greatly appreciated!