Sounds like you got lucky with that one.
Mine has alittle whistle IIRC its a GM4 with homemade turbo master set at 12 PSI, 2.5" crossover, 3" downpipe, 3" to 4" increaser, 5' of straight 4" 90* turnout in front of the duals.
The most whistle Ive heard on a 'road' truck was my buddies 06...