The groaning tank cap breather? It seems very common with 3XVs - just air being pushed out of the tank through the breather as the fuel expands as far as I know. You can often provoke it by shaking the fuel about a bit. I don't know if it's to do with corrosion in the breather as they get on a bit (a bit like all
our groaning I expect...
) or if they all did it from new? My TDR does it too, so maybe it's just a feature of Yam tank caps of the period?