Options>Adapter Information
A box will pop up. At the top, there will be some BIG letters, probably the name of your video card in a box within the box. Over on the right of the box is some arrows. You can scroll through all PCI devices with those arrows.
In the middle of the big box, you will see a small box labeled "Latency", THAT is the PCI Latency Timer adjustment. Your video card will for sure have an adjustment. You can tell IF a device allows you to adjust it's timer because there will be a number in that box. Stuff like your USB on the motherboard probably will not allow any adjustment, but if you have a USB PCI card, most likely it will allow adjustment.
Your firewire card SHOULD have an adjustment. It has for me on three different firewire PCI cards.
The trick first is to write down all settings before you go messing with anything.
My MSI GeForce FX 5200 video card has a stock Latency time of 248. That is a very high value. I might go ahead and lower this anyway, and I don't video heavy games on this computer, thus, the card does NOT need that much time on the PCI buss!
Just lowering the timer on your video card might do the trick, and at first, JUST do that. Lower it by 1/4 of it's value. If you notice improvement, but still some crackles, try a bit more. Do it JUST until you stop getting crackles.
You might need to give your firewire card more time on the buss, thus, increase it's value. If lowering the video cards time doesn't do anything, put it back to the stock setting and try adjusting the firewire card.
If just lowering/raising one of the two doesn't do anything, next try combinations of raising the firewire and lowering the video card.
This worked great on a system I used at a studio that had firewire, usb, Radeon video card (yuk....that card sucked ass!) and a UAD-1 card! A combination of lower the video card, raising the firewire, and raising the uad-1 was the ticket. It took a couple of hours of messing around with things to make it all click, but it worked.
Be advised, you MUST run Powerstrip for these adjustments to work! They adjust this stuff at the OS level, but don't change the defaults EXCEPT when the software is running.
If this works for you, you might consider purchasing the software. Each time you reboot, the "nag screen" adds one second of time that it stays displayed before you can close it. It starts sucking when the nag screen is up for a minute! LOL
Good luck.