most often the noise is not from the fan motor... it is the vibration either between the fan and the mounting points, or the vibrations transfered through the case.
double sided tape will work longer than most of your components will...lol
you dont have to spend alot of money on grommets or anything either...
If you use heat glue, you can usually just peel it of if you dont like it or want to replace it...
if you use it to insulate the srews and mounting holes it will last as long as you want...
cutting out the
"punched hole" areas in front of the fans will reduce noise and improve airflow as well...