it may very well be down to a latency issue but if not id seriously check out how youve got them plugged in.

ive had the same speakers for a while now and when i first installed them into my set up i was noticing a slight bit of crackle every so often that i couldnt find the source of, turned out to be power related from a flouresant light in the kitchen downstairs being switched on so i moved them to the second plug socket in this room and theyve been fine.

ideally you dont want them to be running on multiple adaptors (say a multi adaptor coming from the main socket then a double adaptor into that etc) or things like that in the power chain either, ive got one 6 x adaptor coming from one outlet and each speaker has its own socket on that adaptor.

check whats in your plug sockets aswell, lamps, electric heaters etc can all cause little crackles and pops in the power which become vey noticable through speakers sharing it .

im saying all this and it is probobly stemming from your pc but its worth investigating if you cant find a cure for it