Kohler Pompton K3400PB -- replaced guts and now it's loud

[forgetcolor]

I purchased and had swapped out new tank guts for my Pompton with all new Kohler parts. It works fine in terms of flushing, but is very loud for the first segment of the flush. I would describe the new sound as a pipe hiss, happening during the first part of the refill process. After it's run for a bit, then the hiss goes away and the rest of the tank refill proceeds quietly. Any idea what's going on here, or if there's something I can adjust to fix this?

 

[Terry]

The Pompton was designed for a 1/2" supply line. They use 9.5 gallons to flush.
Sometimes the noise can be at the shutoff at the wall. The old rubber seal can vibrate. I open the valve all the way and back it in just a bit.