You won’t be able to get rid of this 100%

Here’s what I did: removed drywall, filled every tiny hole with acoustic compound, and put in noise proofing insulation everywhere. I bought noise dampening blocks that you can attach to your ducts, but I’m not sure they do much.

Next, if I know I need quiet, I’ve put magnetic vent covers on the vent, which helped substantially. But, then you’d need to remember to take it off afterwards for proper airflow, and preventing carbon monoxide buildup…