If they are gnawing inside ductwork, what exactly are they gnawing ?
It is all metal...
It may be a blessing though, uncork a floor vent nearby the gnawing & drop a bug bomb in, then cork it back up.
Maybe that'll run them out of the duct work ?
Then again, Led Zepplin should work too !
Stick some music down there for them...........
If ya gotta cat, you could stuff the cat in the duct work !!!!!!!!!!!!!
Then you have to crawl under the house & see where they are gtting in...ducts should be sealed.