Thanks Doug,
This is looking towards the furnace about 10' away. This register is the only one I feel air blowing out of.
This is looking towards the rear register about 6' away. Sorry for the poor quality. I see the foil tape blocking the path. I'll clean that up.
Unfortunately, the registers run left to right. It will be difficult get anything down in there.
There are two round flex ducts running to the front bedroom and basement. I can barely see them behind a panel. It would take some effort to get my hands on them. I doubt I could fit my big self through the basement hatch.
Would I learn anything from cutting a window in the coroplast under where the duct connects to the furnace? Maybe the flex duct disconnected or pinched? I'll be working right above the axle.