Without seeing this area, It sounds like rot.
With no flashing at the deck since 1978.The deck is pulling off the house. The nails holding the joists to the band joist are most likely gone.The heads of the nails and the exterior of the wood have probably decayed away.
Looking from the inside you may not notice it that much, but when your carpenter pulls the deck ledger off the house , it may be worse than you originally anticipated.
The good thing about it is you will finally get it done correctly(keep us posted).And no deck collapsed, that was most likely next.
The popping in the winter may have been the deck moving away from the house.From water behind the ledger board, Freeze ...thaw....wieght of snow on deck...wind... any other thoughts?
Let us know what the carpenter thinks is the fiX..
Inspectord