I have an update, I got so sick of the noise that I jacked up my car and crawled under for a look. Three seconds later BINGO. The aft part of my heatshield just before the muffler had literally rotted away away from the bolts holding it up against the undercarriage of my car. The only thing...
Your muffler is rattling from a loose connection near the front of your car. Simply grab the exhaust tip of the muffler next time you hear it. If it goes away get your muffler connections looked at. Cheap fix.
Loose muffler most likely, I have the same problem and it's ridiculously annoying and embarrassing the muffler tends to resonate sounds loudly . Next time you hear it when parked. Grab your exhaust time and hold still. If the noise stops there is your problem
Have you considered, that the sound you are hearing is a loose or damaged bracket on your muffler? I have the same problem and finally discovered the cause. However the muffler is on the right side and your problem is on the left. Vibration sounds can be hard to find due to the fact that loose...