I had a similar issue with 4" inline fan that was in a closet hanging from bungy cords....I had a speed controller as well but due to heat issue I had to run full speed pretty much anyway. Here is what I was able to do, since I am in a closet and able to duct to attic I ended up moving fan to attic, bought a rubber made stuffed it with pillows, set fan in there and even bought an exhaust silencer for my fan....cut holes in rubber made for ducting and silencer and rigged it up. Never had issue with heat or fan shutting off in the attic and it made much less noise. You can still hear it though....Is the fan to get the heat out mainly or to filter out smell through carbon filter....or both? Another thing you can do as I am looking into it now is change your light! I am looking into LED that runs much cooler and I wont need to the big exhaust fan. Although you still need good air movement and a fan to pull air through the carbon filter.