Knocking sound

[iDiaz16_]

Have a 97 Tahoe 2wd and i was wondering whats causing my truck to make knocking sounds as i step on the gas. I can hear it especially in tight spaces or when driving next to the contention wall in the freeway or when accelerating while under a bridge. I know my truck its not a new truck but i have heard older trucks (my dads 94) and i sounds smooth and quite. Sounds like it comes out from exhaust. What is causing this sound? Would it help anyhow by upgrading to Flow Masters? Or some nice exhaust system?

 

[iDiaz16_]

any advices? what exhaust would be better for my truck? i was thinking maybe masters 40s series ?? or what would sound smooth when accelerating. not too loud but smooth?