Thrumming is a new word to me, I thought it may have had something to do with guitars :-)
But no, it is the art of knitting rovings inside a garment for added warmth.
There is a great tutorial on the Yarn Harlot's blog
This is a picture of a pair of mittens from the blog post. One right side out, the other is the inside. That must be so very warm.
Grandma Punk's website has some interesting info about thrumming too:
This pic is from Knit me a River blog
There is so much great information on blogs , now I have another project for my beautiful rovings.