Hi,
When I was recording some DI guitar signal with Roland Quad Capture interface, I noticed that the noise is very loud when idle, even if I turn volume knob on my guitar all the way down. The gain knob on Quad Capture is about 10 o'clock (~12dB). The noise makes the chords cannot be sustained 'cause it's noticeable when the guitar's volume becomes low.
When I record with POD HD500, the noise is very low (Gain max in mixer stage and no noise gate).
Here is the link of samples when they are both idle:


Can anyone explain this?
 
Here is the setting I've used for amp sims:

I've just found this thing: when I switch phantom 48V either on or off, the noise disappears for a while (~4 seconds) before it appears again... What is going on here?

You should never use phantom power when patching anything but a mike that requires that option, you can damage things. 
 
Are you hooking things up the way they tell you too in the manual? 
 
The Hi Z input is for guitar. 
 
Is there noise when nothing is plugged in? 
 
Have you engaged the compressor or other efxs? 
 
If there is no noise when nothing is plugged in, that tells you the unit is working properly. 
 
If the noise happens after you start adding amp sims and stuff then you are just hearing what happens when you add a lot of weird gain stages to you signal chain and don't have it optimized. 

Cactus Music
You should never use phantom power when patching anything but a mike that requires that option, you can damage things. 
 
Are you hooking things up the way they tell you too in the manual? 

 
I don't use phantom when patching anything and I do as in the manual .
 

Cactus MusicThe Hi Z input is for guitar. 

Yes, of course! :D
 

Cactus MusicIs there noise when nothing is plugged in? 

Yes, when nothing is plugged in, the noise is still there! I use many of high gain amp sims software and the noise does not reduce. On the last test, I used TSE x50 v1 and TS808 with all setting set to default. The noise when nothing is plugged in was about -35dbFS, which is very loud!
 
 

Cactus MusicHave you engaged the compressor or other efxs? 

I've used no FX. I always do it when I'm mixing my song .
 

Cactus MusicIf there is no noise when nothing is plugged in, that tells you the unit is working properly. 
 
If the noise happens after you start adding amp sims and stuff then you are just hearing what happens when you add a lot of weird gain stages to you signal chain and don't have it optimized. 

 The problem happens when I add amp sims and other stuffs. But amount of gain is little. All gain knobs in my amp sims/distortion are at 50%, gain knob on Quad Capture is at 9 o'clock (about 10dB). I've used exactly the same settings of these amp sims with my POD HD500 before. And of course there is some noise but not noticeable (~-60dbFS at higher gain!!)
 

This is the demo of Quad Capture when nothing is plugged in...

 
...and HD500 with the same settings:

 
HD500 with more gain:

 
HD500 settings:

 
 
When comparing, you can see the gain of preamp on the Quad Capture is even less on the HD500. The noise is reduced but still there when I turn the preamp of QC all the way down.
 

OK so this is getting somewhere, Sorry I asked all those questions but it's good to get even basics out of the way. 
So what you are saying is that the device sounds perfectly fine until you try and insert the Sims in the signal path? 
In the real world certain stomp boxes and definatly amplifiers add noise, so the traditional method was to put a noise gate on the end of the signal chain. certainly a band aid but sometimes there is no other option. 
At this point it would be great if another owner of a quad capture chimes in. There are certainly a bunch of them around. 
I would recommend looking or even posting in the hardware form for other people who use the unit. I don't think many people visit this part of the form. 

In the meantime, 
Looking at your settings I think what I see is your turning all your outputs to max? This would defiantly be the most noise. 
 
The best "low noise" route is to first have your interfaces input optimized. 
Remove all inserted items and play your guitar at full volume aggressively. 
Set the input gain control just below the threshold of clipping. 
 
Then as you add the sims start with all outputs on the lower side first. It's the input side you turn up, not the output. 
 

Cactus Music
In the meantime, 
Looking at your settings I think what I see is your turning all your outputs to max? This would defiantly be the most noise. 
 
The best "low noise" route is to first have your interfaces input optimized. 
Remove all inserted items and play your guitar at full volume aggressively. 
Set the input gain control just below the threshold of clipping. 
 
Then as you add the sims start with all outputs on the lower side first. It's the input side you turn up, not the output. 
 

The outputs was set to max with my HD500 and the noise was not noticeable. Setting with Quad Capture is in the first picture. After I inserted them on my track (x50 with default settings), I cranked TS a little bit to boost the input with very little of drive (in the first picture). The noise is very loud! Again, look at 2 first pictures to see what I say .