Sounds like the operation of your bathroom fan is generating considerable RFI when it starts up. That RFI can be conducted on the AC line or transmitted through the air. Most AV gear is pretty good at squelching RFI on the AC line. So, my guess is transmitted RFI is causing your problem, although I wouldn't put it past the power supplies furnished with the HDMI extenders to be part of the problem. In any case, the best way to deal with RFI is to eliminate it at the source. Could be as simple as tightening a connection or replacing a switch. Could require replacing the fan. What make and model fan do you have?