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You are all marvellous - (so glad I am being taken seriously - thought you might all say I was going mad!!!)
Can hear it best in dining room and bathroon and does sound like it comes from the loft (it's a bungalow, so has huge roof void) Can't hear it outside at all.
Like the car fob battery running down - never had this happen before but suppose it could be that.
Have checked loft and can't find anything there. Bizarrely it stopped for a while earlier, then came back, then we had a huge storm and now the storms gone it's still there!!!
Perhaps I will never find it - and if it's some battery thing somewhere presumably it will run out eventually and it will still be a mystery!