There are several places to check before looking at the muffler. The first place to check is the joint between the exhaust manifold and the front pipe (this is the pipe that leads to the catylitic converter). There should be three bolts. Make sure the nuts are tight. Next check the rest of the bolts and nuts along the rest of the pipe. If this doesn't fix the problem, you may have cracks in the manifold. To determine if this is the case, you will have to remove the heat sheild covering the manifold (you may have to remove the air intake canister to get access all the bolts). Visually inspect the manifold for cracks and holes. There could be hair line cracks and may be hard to spot. In this case you have a couple options to consider. One, if the crack isn't to big or too wide, you can probably have it welded closed. This is the cheapest option, but won't last as long. Two, replace the manifold. This is okay, but will eventually end up cracking again. Lastly, you can replace the manifold and front pipe with a set of headers. We will be happy to recommend a manufacturer to you.