Hi,
Just a suggestion. (Its a long shot but still). Take your car to a Muffler Shop and have them check the "Heat Shields". Sometimes the bolt on these come loose or the Heat shield themselve are cracked and make an heavy sound (in the beginning) and then a shrieking sound when its gone real bad! Have it checked.
Maybe u decide to put in a free-flow exhaust while ur there and get rid of the heat shields altogether! Let me know what happens.
Good luck and drive safe. Buckle up guys!