It turns out many people seem to have the same kind of problem, the symptoms are the same (pops and clicks in audio) but the causes are different. I did a simple search on "pops clicks" in this forum and got many hits - yes I should've done this before posting :)
Anyway, Windows Vista and 7 seems to have issues with latency. And high latency produces pops/clicks/stutter etc in the audio stream in Windows.
I don't entirely understand the problem but it might be related to resource conflict, drivers or HW. I used the DPC latency checker (http://www.thesycon.de/eng/latency_check.shtml
) and for my Win7 x64 system it reports a latency below 500µs in general but every 5 seconds I get a spike of about 4000µs. And every now and then I get a spike of about 23000µs! This seems to correspond with the GPU throttling.
According to some threads and DPC latency checker, one should start hunting down the culprit by disabling/enabling resources and drivers in the Device Manager, and toy around with active services in Windows to see if this helps latency. After some hours of troubleshooting, changing BIOS settings and changing settings in Device manager, I finally found one issue. It was the sensor monitoring software in the ASUS AI suite II which caused the 4000µs spike every 5 seconds. After I turned the monitoring off they disappeared. I still have the 23000µs spikes whenever the GPU throttles up or down and there is the click/pop. I'm still to find a utility which can turn of my EN210 GPU throttling, because not even ASUS' own overclocking utility SmartDoctor can change the clocks on´my GPU. I've also tried MSI Afterburner with no effect.
And to answer some of your questions:
- I checked for ground loops, there are none
- BIOS has no setting where I can manipulate the GPU throttling and I can't find a utility that works either. The GPU is an ASUS EN210 SILENT, it's about a year old low performing Nvidia card.
- I've tried changing CPU throttling/sleep states but this doesn't help
- I'm going to disable the GPU driver and see if that helps. I'm also going to see if I can move the gfx card to another PCI slot.
Just to show what I'm talking about here, I've enclosed a screenshot. There is an audiable click/pop at both latency spikes. Note this is with the ASUS AI suite monitoring turned off, so all that remains is the "GPU induced" latency.