I had the same issue and it drove me nuts. Took me 2 solid hours but finally figured out it was a monitor program I had from Asus for my motherboard, that allowed overclocking, etc. Caused a spike every 5 seconds and every minute one of those was enough to clip the sound. Drove me nuts!
good luck with it
found out the program that was doing it. It's called EasyTune6, something that came with my motherboard and is used for overclocking, thermostat, fan control, etc. It was set to monitor my temperature every 3 seconds which is what was creating all the annoying, yet consistent spikes...
Yup, been there :) Glad it got taken care of. Hopefully no more drops!