It could be a leak, but it is hard to diagnose over the internet. Just try and imagine the sound of exaust gases being pushed through a small opening and bam, thats what your exhaust leak should sound like.... easy, isn't it. You might only hear the leaking when it is cooler because the metal hasn't had a chance to fully heat up and expand.

There will be some resulting power loss with a leak, although it might be to small to notice now. Try getting up near the wheel well or under the car if you can so you can try and hear if there is a leak or not. You can also try looking at the connecting points for your exhaust pieces, you might see some black sooty deposits from the escaping exhaust gases.