I'd also recommend switching off EVERYTHING that is not needed.
 
Run as bare bones a system as you can - pc, monitors, interface &, er, monitors. Nothing else. Try to ensure that all mains cables are connected as close as possible to a Star System so that everything only comes out of one wall outlet.
 
If this DOES alleviate the problem you can identify the culprit by plugging stuff back in, one at a time until the noise comes back

Slightly off-topic, but...
I recently discovered that any project containing the softsynth GSI VB3 (Hammond Organ) emits a constant pulsating noise even when idle. It is only noticable when the volume is turned up high and disappears when the synth is turned off. Also I believe that the UAD EMT250 reverb is hissy when idle, although I've never checked this out. Maybe there are other noisy plugins too.

Bristol_Jonesey
I'd also recommend switching off EVERYTHING that is not needed.
 

that includes air conditioning, heating, cell phones, burglar alarms, missile defence systems, news on politics, news on economy, vegan cooking, divorce lawyer communication, ... 

Bristol_Jonesey
He's got a missile defence system in his studio ????
 
Wow, way to go.......

if you haven't got one yet, I'm sure you can get one online real quick ;-) sweetwater, thomann, amazon, ... just don't fall for the next monthly subscription fee

This is a grounding problem in the PC or one of the components. This can be hellishly difficult or impossible to fully solve since it can be a combination of issues causing the problem.
My old DAW had this issue where I would get RF noise leaking into my signal through the audio interface. In my case it definitely had to do with some Roland gear I was plugging into my Audio interface. 
 
I ended up reducing the noise a lot by switching off all dimmer circuits as well as using ferrite cores on all cables in the system. I also use a HumX which works fairly well. However even with that I would randomly sometimes pick up weird RF noise. It turned out that there was a radio station within 5 miles of my house. I had an electrician come over once and he told me someone complained of voices in their house - it turned out the signal was so strong that wires in the house were actually inducing and acoustically playing back radio. I didn't even know that was possible :-P

My OLD DAW (AMD 8 core with Gigabyte Motherboard) had the same issue.  I never could get rid of it.  So I made the 8 core my "Media" server with 2 TB Windows storage drives.  That is where I back everything up.
 
I was using the i3 (ASUS motherboard) as the Media server.  Just repurposed both of them and all is quiet now!
 
 

JustGotPaid
This is the new plan. He's going to use one of my MacBooks and start with Logic while waiting for SONAR to come out for MAC. If he can get SONAR to work good on the MAC then I hope his problems are solved. A Windows laptop was discussed and was the original plan, but I cautioned him that there is no guarantee that any Windows laptop will work, but it's practically guaranteed that Logic is going to work on a MAC, and since SONAR for MAC is scheduled for release any time now, he's changed his mind to try the MAC. It's mine and he can use it all he wants for free until he sees if all this is going to work. That's the safest route.

I've been running SONAR on a stock HP laptop since 2009. I don't understand your concern.

Makes sense!

I have a tablet that runs Sonar Platinum. It has a 4 core Atom CPU and only 2 GB of memory running Windows 10 UA. I have a Laptop a Lenovo Thinkpad X220T that has an i7 dual core with hyperthreading and 4 GB memory running Windows 10 AU. (I will be increasing the memory to 8GB soon and upgrade to 64 bit on it.) This laptop is a full fledge laptop and a tablet with pen and touch input.  It runs Sonar very well indeed and my custom built by me desktop running Windows 10 AU 64 bit with a Q6600 and 8GB memory.  
 
All of them run Sonar well. The X220T is good to go for remote recording with a proper multi input USB audio interface. 
 
Nothing wrong with a Macbook but one can get a great laptop running Windows that is a fraction of the cost especially if you go with a refurbished one as I did. 
 
 

I agree with John and Jim, and many others here. I agree with the "great computer" notion of MS Windows. I deplore the world of Apple, but like many, I tried it and it was always an uncomfortable foreign language in an uncomfortable foreign land.
 
Jim, my disclaimer was "basically guaranteed!" Haha. I think Logic on a Mac comes pretty close to that. His frustration level is out the roof...and I was afraid he was going to throw that snack-bit computer off the roof too! He finally gave it to his son the other day and he and some college buddies are going to change mother boards and trick it out every way they can in hopes of making it a gamer. They are a good age to attempt that and will probably do something that will make them all look like a genius.
 
Basically he's going to borrow my MacBook and try Logic while we wait for a Mac-ready version of Sonar to be available. The MacBook is several years old and it will be a friend-to-friend discount just to help him get back to recording. That's the only free route for the interim, and Logic on a Mac is the safest bet I know of when it comes to compatibility. If he can get his paid version of X3 Studio (I think it's called.) to run good on the MacBook I'll sell it to him cheap.
 
Ideally, we both want to be on a Windows laptop with Sonar so we can carry our recorders back and forth when we visit. Portability has always been very important to me, and he sees how convenient it is, and we live 15 miles apart in different towns, so we have to travel some with our song projects.
 
Totally off topic, but do we have any Nashville SONAR users on the forum here?
 
I'll keep you updated, and I appreciate all of these really interesting ideas, experiences, and discussion on all of this.
DS

I had exactly the same problem. I bought a ground isolator , put it between my audio interface and speakers and the problem was resolved. Before installing the isolator I had to keep the volume on my monitors very, very low for them to be of any use at all. Now I can turn the volume up all the way without problems.

Hmmm... a crappy USB cable????? Never thought of that.  Thanks for the info.
 
Jim