Metal socket touching the metal case

[TNb]

I have a problem with 1/4" jack socket - when I install it to the drilled hole in the case it obviously touches the case. While it is not causing any problems in functionality(this is passive attenuation box for audio amplifier) of a device it causes problem if you touch the casing - it makes high pitch sound.
What can I do about it? One way is of course to use plastic sockets, but is there other solution?
Or does it has to be like this and then you just paint the casing and it kinda like eliminates the problem? What manufacturers do in this case?
And by the way it also goes about the potentiometer holes touching potentiometer casing(which I don't fully understand since pot's casing is not connected to any wire)

[Paul Price]

I think you've miswired the phonojack and reversed the ground and signal leads, otherwise touching the supposedly grounded box would not cause any squeal.

[TNb]

Paul Price, actually it doesn't make a difference since I don't have a ground here, it's passive device. Something like in the pic below. Amp output is from transformer, so it is floating.

[EvilGeniusSkis]

could you be coupling in mains frequency? I've had that happen when touching the TRS input on computer speakers.
even a crappy frequency counter on a cheep DMM should help confirm/eliminate this.

[Whales]

I would expect the connector is designed to connecting the shielding/return/ground wire to the case.  If so you should not be experiencing these 

Is your signal perhaps balanced?  If so make sure you are using a 3 wire connector ( +, - and ground) and the wiring is correct so the case is attached to the ground, not one of the signals.

[Richard Crowley]

Actually the input jack SHOULD contact the metal case to complete the Faraday shielding around the entire circuit.
The high-pitch sound is a symptom of some other problem that should probably be addressed.

While I wouldn't recommend it here, it is customary to use a "shoulder-washer" if you don't want the metal barrel of the jack to make contact with the panel.  Or to use a jack with a plastic barrel.

Here is a photo of a shoulder-washer on a 1/4 inch phone jack.  Of course, you must use a flat insulating washer on the other side of the panel to complete the isolation.

[EvilGeniusSkis]

TNb could we get a picture of the jack on its own as well as some pictures of the whole device.

[Frink42]

I think you have a ground loop. and when you touch the metal case you're coupling in some noise, not so much because of actual coupling but rather because your ground is floating to whatever you're floating to. So, ground loop: not all "grounds" are the same in your circuit

I see two possible problems(and solutions) here:

-You're not soldering anything to the "ground" (metal ring) of the jack. Your standard 6.35mm jack has both a "live" wire and a shielding. The shielding goes to ground (the long part on the actual jack connector). Since you didn't solder anything to the jack, you have a floating ground there. In some cases, if you get lucky, the case itself will connect all ground pads with a really low resistance path, but believe me, I've worked with high gain (guitar amps) circuits before and it's better to have all grounds connected to one point.

-If you really, really want to insulate the jack for whatever reason, then you can get an insulated jack like http://www.taydaelectronics.com/6-35mm-1-4-stereo-insulated-switched-socket-jack-solder-lugs.html(that one is stereo, but you should be able to find a mono version. Or just use the stereo and forget about one lug). That would insulate the jack against the casing, but I believe you'd still have the ground loop problem unless you put them all in one point.