do you have heat wrap on your pipes? if so can heat wrap burn if it get's old?? Are you sure it's a fibourus smell, and not oil (or something) burning?

The only other thing I can think of, is liquid gasket/gasket sealers, they tend to smoke after you use it to whack and exhaust together. This would only apply if you'd recently sealed the exhaust.

Hopefully that might help dismiss a few things or something. hope it works out.