I put the MOTU as far away as possible from PC and power cables, but no change, still get the static crackle noise from one of the monitors.   

I'm clueless when it come to "grounding" - is there some kind of box I can get which might help with this?  If anything the noise is getting worse now (or I'm getting fixated with it :-)) - I really need to find a solution as basic monitoring is  pain, and I cannot use any external hardware instruments (guitars, mics etc).

I'm not sure also why its only one monitor that is affected.  Its the one closest to the PC, but moving the PC further away didn't seem to help.

It's hard to say, as it seems like we have discussed the obvious stuff.

It seems like you mentioned that it happens in both your speakers and your head phones... and that is why I have been thinking it is on the input side.


Now, did you just mention mics are problematic as well? That adds a new twist... 

I am beginning to suspect that you are observing various noises that occur for various reasons and that each issue has it's own solution. This makes it more difficult to trouble shoot as it takes a bit of experience to isolate each cause/effect.



Let's start with some basics.


What is you computer plugged in to?

What is your display monitor plugged in to?

What is the MOTU plugged in to?

What are the speaker monitors plugged in to? Are both speakers plugged in to the same power outlet?





Also, if you physically move the speakers does the problem stay with the monitor? (this is a variation of swapping the cables, but also considers whether the noise is being introduced inside the speaker)


Are you sure that the noise issue you hear on the head phone is the noise in the speakers or is it possibly two different noise issues?




You may want to post sound samples of the noise. Some one may recognize it's source by ear.


best regards,
mike

Everything is plugged into one plug adapter which then goes into the wall socket.    

I physically swapped the monitors around --- the same monitor still gave the noise.   It does appear to be though a PC sound - its louder or softer for example when opening/closing PC programs ...

I don't hear the noise when monitoring through headphones - unless I plug something in like a synth or guitar.   I'm not sure that is the same noise or not ... I tried to record the nose from the monitors but that did not record, but with a guitar, it records the noise along with the guitar sound.  

It could be the power cable. Or a broken ground connection at the input. Are you using balanced cable between the MOTU and the monitor (1/4" TRS to XLR)?
The problem exactly follows one of the two monitors, i.e. none of the cables?

Wilko

The cables I am using between MOTU Ultralite and monitors are PRO CO  StageMASTER 224SM - XLR connection.   I changed cables around and I changed monitors around - the issue follows the monitor, not the cable.


I changed the power cable around on each monitor and still the same monitor had the issue.

It sounds like the one monitor has a problem.
What you could try is feeding a signal from another source to the monitor, like an iPod or CD player.
Is there a chance you can take the monitor to the dealer to have it checked out?

Wilko

I agree with Wilko about the monitor diagnosis.

And I suspect that you are becoming aware that the other noise associated with the guitar is another noise.



BTW, thanks for actually answering all the questions I ask... it's the only way to help when we aren't there with you... but I realize it can seem annoying. Thanks for making it easy by actually answering them.


I do have another question; What sort of guitar? Humbucker? Single coil?



best regards,
mike
 

Just a quick general update - I turned off the PC, and the MOTU, and plugged synth direct to monitors using 1/4 inch analogue - first thing I noticed was how quiet the room was as my PC is so damned loud just being turned on!  The monitors do give off a bit of a hiss, though not annoyingly loud, and pressing my ear right up I can hear a very quiet hum - I cannot hear that from listening position.   There was none of the static/noise/crackling etc I heard when the PC was on ... then I turned the PC itself back on, no noise - it was when I pugged the firewire cable into the MOTU that the static/noise/crackling came back.  Don't know if any of that  that means anything ... 

The guitar?  Hmm, I am no expert - its my daughter's guitar - www.tokaijapan.com     a GS100


And, thanks, for your help ! 

I had a vary similar situation with a M Audio Fast track pro. 
There was all this background noise and it got worse when I used any unbalanced cables. It was still there but about half as much when using a Mike cable. 

The noise would die the minute you toggled Sonars Audio engine off. Try that. 

In the end I just used the SPDIF and thank goodness I own a digital mixer and it was then dead quiet. I think it was A/D converter related. 

The only mystery after reading this is why one of your monitors seems to be acting up. But you say when you test them stand alone they are fine? Your audio interface might be the problem.