I know EXACTLY what you are talking about. The same thing happened to us. Since i have no idea how it was fixed, i just asked my husband who is plumbing handy. He said in our case the chain was too long so that the plug wasn't sealing the bottom effectively. But it could also be a worn out seal (so he says). Either way, the best fix is to get a new apparatus and exchange it out. According to him it's pretty easy and a 10-15 min install.
Hope that helps :)

That sound may be from rats trying to crawl up the main pipe and into the toilet. For the love of gawd, duct tape the lip and sit to the bowl so they don't get in at night while you sleep ..!

Naw, just kidding.

This could totally have something to with the seals wearing out at the bottom of the tank and letting occasional water through, in which case, yanking and replacing the whole apparatus should do the trick. It IS pretty easy ... just follow the instructions that come with the new assembly. Total cost is anywhere from ten to twenty bucks, depending on whether or not you splurge and go for a new ball and flapper.

BUT ... I've seen (and heard) some of the newer, higher-tech, fancier toilets that make this sound (or something similar) from the top of the fill pipe. There are a lot of different toilet types these days (including over-engineered ones that use electric vaccuum devices to shoot your waste out to sea), and they can make all types of crazy noises when they start to develop problems.

If this happens every five minutes, have you considered just mixing yourself a drink, pulling the top off the tank and seeing for yourself what is happening when the KYUSSCH happens? Hell, make a video and post it to YouTube ... it's be a lot more illuninating than a lot of the crap that's been on there lately ...

Good luck.