I'm trying to reduce fan noise with my i5 5425 and I read that turning off hyperthreading might address this. I went into visual BIOS and looked at every screen and did not see where I could do this. Early this morning, I emailed visualbios at intel.com and asked how to do this, and haven't yet received a response; I wonder if I might get a quicker response here.
Since I didn't get a reply, I engaged tech support. Alberto at Intel informed me that hyperthreading cannot be turned of in the BIOS (or anywhere else), so this issue is now closed.