I'm almost sure that what you're hearing is a fog horn on the river. I came over the Penny bridge yesterday afternoon and was surprised by the very low cloud to my left. It covered the top of St Hilary's Church. and was rolling down towards the Bidston recycle plant. In fact it was so localised, I thought it was smoke from a fire on the Landfill park. Then when I was retiring for the night, I could hear the fog horns from my bedroom. It's three short bursts, then a rest then repeat.