Thanks for ideas.
The first thing I did this spring was to replace the center-mount and I also looked over the spring rubbers, the right lower was missing so installed a new one.
The sound from the rear was worse before I replaced the upper center-mount.
The rear greasing helped for a short while, probably the lower rubber mount to the vertical support is all warn, if that’s the case the center rear ought to fall down a bit to cause the center spring to ride on the tank bracket.