My Panda does this, only in wet weather, usually after a deeper than normal puddle. Some time ago, I found some exhaust mountings looking soft, so replaced them, and the noise seemed to be cured.
But Saturday, the noise arrived again, after a large puddle. All exhaust mountings are nearly new. Under the Panda there are plastic covers over the brake lines running some of the length of the car. These are all secure, but I'm now wondering if the water disturbs them, and then the airflow takes over causing them to vibrate. I don't really want to remove them, and then run through puddles, so am currently pondering how to test this as a theory.
But Saturday, the noise arrived again, after a large puddle. All exhaust mountings are nearly new. Under the Panda there are plastic covers over the brake lines running some of the length of the car. These are all secure, but I'm now wondering if the water disturbs them, and then the airflow takes over causing them to vibrate. I don't really want to remove them, and then run through puddles, so am currently pondering how to test this as a theory.