No disrespect intended, but how good of a shot are you? because if you are shooting foxes at less than 120 yds, I assume some will be 60-80yds ect
You won't get any quieter than a .22 rimfire..........and at the sort of ranges if you have good shot placement it will be enough for the job
The HMR will still give the sonic crack even with a mod, only you know what noise is acceptable