mystery liquid and rattling

You've got nothing to worry about but drive the car more... Sounds like condensation in the muffler. Most mufflers have a tiny hole in one end on the seam. That hole should point down but not everyone knows that and if it's up theres a good chance theres water stuck in it. BTW, not the best idea to taste it... If it was coolant (it would taste sweet), even a small amount that you ingest it, you will destroy your kidneys permanently.