21 TRD Pro JBL Upgrade

Perfect. Its usually one of them. Gain issues will cause hiss and white noise that you are experiencing. If it is alternator whine that varies by speed of the engine that is usually a ground or power location issue. Glad you got that resolved.
Also, if you lose volume this way you can usually add back some on the output side. It is truly a balancing act. Glad you got it figured out.
I had some amps that you could hear the noise your referring to when I cut the truck off and before I opened the door there was hiss or white noise. Minor irritant but then I changed amps and took the truck to a friend to tune and he told me
I was probably fine with the amps I had but I had not adjusted the gains correctly. The new amps are silent but he also did an entire tune and we adjusted all the gains again.
Now I have three amplifiers sitting in a box that are only 6 months old and I thought they had noise but turns out they don't. Ill stash them away for another build.