Welder hum when off

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Hello everyone. I read regularly but seldom post. I have a question/problem. I FINALLY got my workshop finished (Finished enough) so I plugged my welder in for the first time since I got it 3 months ago. It was my uncles old Hobart CyberTig 300. It worked great when it was last used years ago. Long story short I got it cheap and how have an AC/DC tig welder of my own. Well tonight I plugged it in for the first time, turned on the argon and tapped the pedal. Everything seems to be fine as far as the pre flow, post flow, foot pedal etc. I even ran my first bead on it tonight. Well afterward I turned the machine off, turned the gas off and began putting everything away. I listened closely and I could hear a low humming sound. I looked everywhere and turns out the welder is making a very faint humming sound, even when I turned it off. I flipped the breaker and the sound wend away. I turned the breaker back on and the sound came back. I tried flipping the machine on and off a few times but same problem. At this point I'm sure SOMETHING is on in the welder even when the power switch is turned off. All the lights are off and fan is off but still the hum. I flipped the breaker off for the night so that something bad does not happen but my question is what could cause this problem? I don't know for a fact but I'm pretty sure it's not normal. The hum sounds like one of the transformers is powered up. Much like one of the power companies transformers in peoples front yard. Aside from the power consumption I don't want to burn anything up. Any suggestions? Thanks ahead of time.
Reply:Water cooler?  Power factor correction?I would turn off the breaker when not in use.DavidReal world weldin.  When I grow up I want to be a tig weldor.
Reply:It is most likely the control transformer, it is what runs all the crap inside (usally at 115Volts), my Lincoln IdealArc does it too. I just unplug the welder or shut the breaker off. If you put an amp meter on the line it will be drawing much less then 1 amp off of a 230 volt line, it would be fine to leave powered up.-Dan
Reply:I think Engnerdan is right; control transformer.Even a very small transformer will hum quite loudly, especially if mounted on sheet metal.Good Luck
Reply:Well thank you to everyone who posted. I will be checking amp draw on the line when the machine is off, with an inductive amp meter. If current is low (Around 1 amp or so) I'll assume that the hum is in fact the control transformer. I hope that it is, and that everything is in fact ok. I have never been around old welding equipment such as this (Early 80's). All the welders we had in tech school were new inverter type Millers. I am just happy to have a TIG machine of my own, no matter how big, heavy and old it is. Thanks again for every ones help, I'll post my finding later on.
Reply:The Hobart I run don't "Hum"..But there a few big things inside that stay warm all the time even if it has been off all weekend.....zap!I am not completely insane..Some parts are missing Professional Driver on a closed course....Do not attempt.Just because I'm a  dumbass don't mean that you can be too.So DON'T try any of this **** l do at home.
Reply:Hello again everyone. Well I checked amp load with the welder turned off and the total current is about 0.39 amps. I believe you guys are right about the hum coming from the control transformer. I guess that this is nothing to worry about. I plan on killing the circuit breaker at the end of a day of welding or when I won't be using the welder again for a period of time.    Other then the little scare I got thinking something may be wrong with it, this welder is amazing. For an old machine I am amazed that every one of the control knobs works. The arc is very smooth on either AC or DC and there is an enormous amount of power available if I ever need it. In school we had some 175 amp AC/DC machines and they were at times slightly underpowered when welding larger pieces of 0.120" aluminum (Often times we would preheat with an oxy torch to make the puddle easier to start). This machine with it's 300 amps available, only limits me to how much I want to spend on my electricity bill.    Aside from the machines large size and 1100 pounds of mass I don't see why more people are not interested in the older transformer machines. I mean if the unit does not need to be mobile and you have the room to store the unit (And can afford a 100 amp circuit to power it) they make great machines and are much cheaper to buy and repair later down the road then the never inverter machines (Not that I'm knocking the inverters, I just can't afford to fix one) Thanks so much for every ones help. I'm going to go out to fiddle in the shop. Everyone be safe. I'll keep in touch.