Running Berks hfc stock y pipe and top speed y back exhaust. I noticed that on down shifts it makes a hissing noise and when I rev it sounds like a lot of back pressure is going back in. Wondering if anyone else had this happen to them and if there’s any solutions.

 

LOL. normal. Haha. Couple ways to go. Back to stock cars or match piping diameters or get a quality exhaust. Only OEM cars are guaranteed to eliminate it. Live with it as it's very common when moving to aftermarket cars or test pipes

 

You could try changing out the y pipe to Motordyne XYZ pipe with resonator or any other reputable y pipe.