Scan for Virus/Ad-Aware... Check your sounds from the control panel and see if one of them is assigned that quaking sound.. never experienced it.. probably because I have most of my sounds turned off except the warning ones.. also maybe put it in safe mode and see if it makes that sound... how often does it happen??
i bet its just some ad or pop up you didn't realize... or perhaps some site you go to with audio ads...
lol... it's prolly that duck shooting ad... hahahah or it could be your harddrive dry humping your dvd drive...
^ Quacking? Website ad! I had moo-ing sounds once until I found out it was a Flash ad within a webpage I was browsing.
lol.. so the quacking was really an ad? haha.. and I was half hoping there was more to it.. lol.. jking