Rattle under car, not exhaust
 

So, I've had this rattle noise under my car. Sounds like it's directly under the engine. It sounds like a hallow piece of metal. My first thought was a cat was going out or a flex pipe. IvI' been to two different muffler shops and both times I couldn't get it to reproduce the rattle. It's completely random and is getting worse and louder. But it only rattles at idle. both shops said it's definitely not in the exhaust. Someone told me it could be the flywheel, does anyone know what ells it could be?

It would be a flex plate, not flywheel. And that wouldn't be random. If it came apart you would know it. I'm suspecting you have bad motor mounts. That's typically what causes excessive vibration at idle. Things that wouldn't be vibrating if the mounts were good may be making noise, but won't if the mounts are good. Use to have the trim around the sunroof and rear view mirror vibrate at idle, gone after replacing the engine mounts.