I was just reading about this yesterday in the kool aid dyeing article on knitty!
noil or bourette silk yarn:
Short fibres containing crushed pupa left behind after making higher quality spun silk are made into noil yarn. The shortness of fibre length results in a lack of lustre and body. This is lower quality spun yarn using the same metric count system as the above spun silk. Noil has the strongest silk odour due to impurities in the yarn. The majority of the smell dissipates after washing, but can return again when wet.
Some silks exude an awful smell when you wash them, often a sign that the fibers weren't processed thoroughly.