All switching-type PSUs emit high-pitched sounds, though most of the time it can't be heard since the fans are usually louder. An electrical humming is also normal. It's clicking since it has no load - the output voltage rises quickly, causing the regulator to cut the power (that's why it's called a *switching* PSU). The output voltage drops and it turns on again, then off, etc. It's actually a bad idea to run a switching PSU without a load for any length of time. Anything of greater than or equal wattage. 250W units are rare now, I think the minimum for a standard-size PSU is 300W.