1. Navigate to [Steam folder] > steamapps > workshop > content > [ETS2 folder]* > [F-Max folder]*
2. Open base.zip with something like WinRAR
3. Navigate to def > vehicle > truck > ford.fmax > interior
4. Open sounds.sui with Notepad (create a backup of this file just in case)
5. Locate sounds[]: "noise|/sound/truck/ecotorq500.bank#interior/noise" and delete everything within the quotation marks (so you'll be left with sounds[]: "" on that line) and save the file

*I'm pretty sure these folders are given random numbers, but for me the ETS2 folder was named 227300 and the F-Max folder was named 1915802227. You can sort by date modified if you installed the F-Max truck recently, but the F-Max folder will have base.zip, base136.zip, info_content.zip (which you can open and check if the mod icon image is for the F-Max), and versions.sii inside of it.