If your getting a second/3rd hand switch, and you don't plan on putting it in a rack or keeping it forever, there is the option to pop the top panel off, examine the power consumption the existing fans on whatever switch you get, and then find a 120/140mm fan that has equal or less power consumption. Then cut a 120mm hole in the top as close to the primary processors are, and then blow cold air directly onto it, allowing the air to flow out through the usual side vents.
I've done this on two separate switches so far (one of them is literally 1ft from my head in my office) and I can barely hear it, and it gets ample cooling (actually gets more airflow then with the stock fans).
Just an option if your ok with hacking the lid/top of a switch.
Be aware though, some switches, this doesn't work as they are made with two/layered pcb's with interconnects, and rely on the cross flow to cool in between the PCB boards.