5.0GTConveritbleGuy

What going on everybody, now ima explain this the best way i can, i have a 89 GT Converitble (5.0) and it has pipes on it, the people who i bought it from said it has flowmasters on it, thats all i know about the exaust. Now i noticed when there hott (Pipes)they dont sound as good and there not has noticable but still get loud at 2000RPM's. When there cold there shoud soooo tight and there LOUD as hell. i dont understand whats going on, are all pipes like this, this is my 1st car and im confused and a little pissed off. So can some body let me know what could be the problem and what i should/could do to keep them loud at all times.
GEt at mE.