Bottom end knock is either internal to the engine and serious or external and possibly less so. If i might suggest, pick up a set of mechanic's ears at a local tool outlet, put the car up on stands or a hoist, run it and start probing things for the source of the racket. If the torque converter is loose on the flexplate, for example, you definitely want to get on this as it'll destroy the trans and possibly the engine pretty quick if left untreated.
On a positive note, it might be as simple as an accessory drive or an exhaust tap, which you'll quickly find with the ears or see with your eyes.
2014 Premium GT, SGM, Brembo, Auto, Tech, Comfort, etc. GT500s, Steeda UCA, VMP auto N/A tune. 100 shot nitrous on BBR tune. Best e/t 11.42 at 120.64
2012 Premium GT, Candy Red, Brembo package, 3.73, MT82, Comfort, Tech, et al. Procal tune, Roush UCA, UMI poly LCA, GT500s, Steeda red bracket, Hoosier 28x10x16 bias drags. Best e/t 11.91 at 115.23 RIP