Bang on about
Talk repetitively and boringly about something.
The term 'bang on' has been used to mean either 'exactly on', i.e. physically close to, or 'exactly right'. Those two meaning are exemplified here, respectively:
An advertisement in Punch April 1936: "Quiet garden square near Hyde Park. Real hot water. Bang on Tubes."
('Bang on Tubes' means 'very close to an Underground station'. 'Real hot water' - who knows? Better than imaginary hot water I suppose.)
John Hunt and Alan Pringle Service Slang, 1943: "Bang on, bomber slang for 'O.K.' or 'Everything's all right'."
These derive from an earlier American usage of simply 'bang', meaning 'exactly'.
'Bang on about' has a different meaning though and doesn't appear to be related to either of the above. It is a British coinage.
The Economist, October 1979: "Mr. Patrick Jenkin - Social Services - In cabinet tries hard but is inclined to bang on a bit."
Julie Burchill, Sex & Sensibility, 1990: "This cry was taken up by the alt coms [alternative comedians] - Benny and Jenny and Lenny and Dawn - who would bang on about Reagan being an actor."