Hiss is not an artifact of an ungrounded audio source. Hum or buzzing would be the result of something coming ungrounded. It is also highly unlikely that anything has come ungrounded inside the computer anyway. I had a buzzing issue with a USB audio adapter once and it turned out to be a bad power supply. I've never had a hiss issue though.

Hiss would be audio leaking in the system somewhere, or a gain that is either turned up in software or hardware. Or potentially you have a bad gain circuit?
As John mentions...ground would be a hum or buzz, most often at a harmonic of 60Hz.

Well i'm pretty sure it has something to do withe the USB grounding issue.

BTW, tried butchmoore's method and it didn't help.

What leads you to believe that it's a grounding problem? The audio interface between the M-Audio USB preamp and the computer is 100% digital. If the USB port had come ungrounded, then no power would be supplied to the USB preamp, nor would there be any USB data transfer to the unit and, therefore, no audio.

I think there is some sort of gain problem as Spot has suggested. Hiss could certainly occur if you had the mic preamps turned all the way up with no microphone plugged in.

John Cline is right on the mark.

An open analog input, perhaps on the computer's built-in sound card is a likely culprit.

John,

It's certainly correct to say the USB audio is 100% digital. But ground problems do exist that can introduce 60 Hz hum and other "digital" artifacts into the signal. Ask any mobile DJ that uses a laptop with a USB external sound device. It's pretty much standard practice to lift the ground on the laptop.

M-Audio recognizes the issue in their Knowledge Base:

http://www.m-audio.com/index.php?do=support.faq&ID=12cfebbf1c694ee8664859cfd004577c

What can be a minor nuisance at home can be a major problem when introduced into a large sound system. Unfortunately, it did not solve Patryk's problem, but it is certainly an avenue to investigate when a noise problem occurs.

OK, now were talking about ground loops, which is different from an open ground and also completely different from the hiss which was the original problem that started this thread. I have spent close to 40 years doing audio and much of it doing live concert sound, so I'm well aware of what a ground loop is. I just don't happen to think that this is Patryk's problem.

You really need to check the M-Audio software mixer application for this box. I suspect that you have max gain on one or more unconnected inputs (out of the four available)..

Don't think that Windows will wait to run drivers for this box until you plug in the hardware, that ma not be the case.

Is your computer a laptop or a desktop?

You are indeed correct, sadly.
Even though the signal is 100% digital on the USB connection that same connection can carry all manner of other nasties that radiate inside the audio device at high frequencies. These get rectified and end up in the analogue circuits and out the speakers or into the mic pres.

I have a bit of problem with my monitors fed from my M-Audio Firewire 410. If they're up loud enough I can hear a sound whenever I move the mouse. This only seems to happen with these Behringer monitors and I'm not the only one to experience problems. Hopefully some small caps on the inputs to the monitors will cure the problem. Failing that I'll try some audio transformers.

Bob.

Bob, what are these: "small caps on the inputs"

TIA

Grazie

Capacitors. Small enough value to not harm audio frequencies, large enough to attenuate anything else. Getting the right values and location can involve a bit of black magic. In general they should be as physically close to where the signal lines enter the device as possible. As you connect them between the signal line(s) and ground you also need to take care with how you connect them to ground. Wires can act as aerials, even the wire connecting the capacitor to ground.

Shure make an inline device, the AR15F which may work very well for getting rid of these problems, or not.
Electrmomagnetic Interference can get into things in many ways, mains connections can carry it as well. Ferrites may solve one problem and be useless for another. Even with a lab full of expensive instruments to help you see and measure what you're dealing with it's still a bit of a dark art.

Bob.

This is the problem when you get a bunch of audio geeks and non-audio geeks talking about a problem sound. Like the CarParts commercial where car owners describe a problem...
If it's a HISS-it's almost always gain related.
If it's a HUM/BUZZ- it's usually grounding related.
If it's a variant noise, it's almost always interference.

Geeks with a long background in live sound are extremely familiar with all of these sounds. Mix Magazine once held a contest at NAMM, "Identify this sound." I'm proud to say that the only person that beat me in a room of about 100 people, was Bob Moog.

Check the gainstages in software and hardware both. Disconnect all cables except the USB. Turn everything down; do you still have HISS? Is Hiss the only 'right' word with which to describe the noise?
Disconnect speakers and plug in cans (headphones). Do you still hear it? A ground loop cannot occur in a USB chain. I'm fairly sure it would have to be combinant with an outside connection.

1. John and Spot are, of course, correct. This is NOT a ground problem. That will give you a 60 (or 50) Hz buzz or hum.

2. It could be a shielding problem which is letting unwanted "stuff" from the computer to leak into the signal. This is quite different from grounding, although perhaps it sounds similar. However, this is pretty unlikely inside a computer, because most connections are via PC boards and the shielding is inherent in how they are laid out. Also, shielded cables don't suddenly become unshielded (although the connection of the shield to ground can become broken, but then you might have a ground loop, which as already mentioned by Spot and others, is not what you have, based on the sound you are reporting).

3. Most likely this is a defective sound card, or perhaps a defective device (like a CD-ROM) but you don't have to guess: There is a very easy and simple solution to your problem (see the following).

What I suggest is that you open the mixer in Windows. Go to the "Properties" and make sure that EVERY control is "shown." Even if you don't have a device in your computer and therefore that device is disabled from opening up in the mixer, you want to, at least for this test, enable it. You should now have a mixer which shows perhaps as many as eight or ten different devices.

Next, MUTE every single one of these devices except for the master volume. Does the hiss go away? I expect that it will. Then, go to each device, one by one, and un-mute that device and turn its volume all the way up. Then, mute that device again, and un-mute the next one.

Somewhere during this process, hopefully, you will find one device that is causing the problem. My bet is that it will be one of your CD-ROM or DVD players, or something similar. I've had this happen before, and I just kept the darned thing muted and did lots of productive things with that computer for many years.

Hehe Spot,
Most non-geeks here (at least those under 50) probably won't have a clue who Bob Moog was.

John Meyer has the correct way to handle the troubleshooting. Ive been in audio business for over 50 years and uf, indeed, the noise is a "hiss", it is not a grounding problem.

"and uf, indeed, the noise is a "hiss", it is not a grounding problem. "

There indeed is the crux of the problem. No one here knows if the "hiss" is a traditional hiss i.e. white noise created by thermal motion within devices such as resistors and transistors. Short of looking at the errant sound with instruments there's no way to know what it is, where it's coming from and how to cure the problem.

I've had to wrangle one "noise" issue which did the exact opposite of what some here are saying. It got worse as you turned the gain down. Possibly the problem had something to do with how voltage controlled amplifiers work, the noise can come in from the control voltage and not the signal chain or the gain being applied to it. In conjunction with the manufacturers of the on camera light we cured the problem. It took a LC pi filter in the DC supply and some care with shielding to tame the problem.

Even balanced audio leads can cary non baseband signals into gear, the very shield on the lead can be what's carrying the signal. So called balanced audio inputs can not be at all balanced at MHz frequencies, even audio transformers may not be perfect, interwinding capacitances quite happily let the common mode RF signals pass.

Reading the OP several times it'd be very brave of me to hazard a guess as to what the problem is. I'd certainly not be so foolish as to state categorically what it is not.

It would seem that Patryk's preamp is powered from the USB port on the PC. He could try two simple things:

Try using a different USB port. If you're using the ones on the front of the PC they have longish unshielded wires inside the case. It's possible the power or ground wire is picking up RF noise and feeding that into the Pre via the power (not the signal) lines.

Second idea. Try running it from it's own power supply.

Bob.

By any chance is this new computer capable of answering a VOIP call without being turned on?
Some of the latest mobos and their BIOSs are Skype enabled. That could mean they're accepting USB audio inputs and applying AGC. This circuitry is On even without the PC being booted.
Only a wild guess but it fits all the symptoms.

Bob.