cant get it to sound right?? help

[loenuf]

i have a 96 x-cab auto w/305. i just installed a flowmaster and sounds great until i downshift. at first it rummbles nicely, but after a split second, something takes over and the rummble goes away. my 88 did the same exact thing. i cant get rid of it. whats causing this and how do i fix/dissconect it? it only sounds good when you dounshift to second, but after one or two rummbles, it goes away! and sounds like crap. someone said modify the smog pump, but i dont have one. thanks guys, this is realy gettin to me......