What causes metal-to-metal grinding sound from tumble dryer?

I had the same problem. Its most likely the bushing that surrounds the tumbler that holds the clothes. When it wears its grinds metal to metal. You can replace it, but in my case, I just decided to replace the whole drier. Sounds funny, but try spraying a little oil or wd-40 into the seam right behind the dryer door between the drier frame and the tumbler.(Its where the bushing is.) It will temp. solve your problem but will only last a little while.