That’s not good at all. It sounds like something is overheating badly. First don't use it anymore until you have fixed the problem you might totally destroy the part if you haven't already (by the time you smell it, it's normally toast). Open up your computer and check to see if all your heatsinks and fans are firmly attached. If they are, then take a close look at your Power supply, they have a tendency to burnt out over time (especially cheap ones). If it has any brownish marks on the outside or inside you will need to replace it. If all of that looks good and has no problems then you might have to start it up with the case still off and make sure all your fans are running properly. Only leave it on long enough to tell then shut it off. If they don’t run fast and strong replace them.