under belly re-install

I have an issue now with one of my ducts in the nose of my 5er. The furthermost duct just has next to no flow coming out of it. It's only maybe 2' of duct work more than the next closest and the flow difference is night and day. They both exit into the basement and 1 turns and has a register in the step while the other turns up to the floor. I think a mouse or something has created a hole in that duct somewhere so I'll have to pull the bottom off to inspect it. My point I guess is a hole could be a real problem depending on what duct it's in. Fortunately I have a rear bedroom which gets tons of heat. :D I would still like to figure out what's going on with that one though because if the kids ever stay up there in the bunks during the winter they'd get pretty cold I would imagine.