Can anyone else here it? Sounds like an engaged tone except it's coming out of a loud speaker!

Anyone know what it is?

Yes I can hear it up in Liscard. Seems to have been going since about 5pm.
Could be on the river The Northwall light foghorn?

It is the foghorns on the
river. The fog has been quite
thick at times for the last 24 hrs.

I live a stones-throw from the Mersey. Heard zilch.

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.

From Wirral Coastguard: