Alright I have a 1999 Blazer 4.3 with a Flowmaster Original 40 series exhaust with 2.50" pipeing from the cat back and it rumbles nice and gets really loud but after 3000 rpms it starts to fade off a little and around 3500 rpms it really goes quite. Not like still loud with a different tone, almost all not able to hear it at all. Im not sure if its because of how fast im going just wondering.

 

It likely is just how fast you are going.