I'm also having this problem. It smells like clutch to me, but i'm not experiencing any slippage(!). Its definetely not tyres and, like you, my exhaust is blowing ever so slightly. I'm not too sure what to do about it, in an ideal world i'd have the lot out, new zorst, fly-wheel and clutch, but i just cant afford that kind of thing at the moment!
Are there any other signs to look out for if you think your clutch is on the way out?