Green’s Dictionary of Slang

a chattering, but not unpleasant, young man; thus a chattering, inconsequential form of speech; the implication (see cites 1801, 1858) is of effeteness, even homosexuality, cf. cake-eater under cake n.¹

[1773		O. Goldsmith She Stoops to Conquer Act III: At the Ladies’ Club in town I’m called their agreeable Rattle. Rattle, child, is not my real name, but one I ’m known by. My name is Solomons].
1780		Mms D’Arblay Diary I 242: [chapter headings] An Evening Party — Anstey— Lady Miller — An Agreeable Rattle— A Private Concert.
1801		Mthly Mirror 11 368: His flattery makes him much admired by the fair sex; and the agreeable rattle of his conversation, and profound knowledge of dress, make him pass for an arbiter of taste. His own dress is religiously exact.
1809		Dublin Eve. Post 9 Nov. 3/3: In the after-piece last night he was an agreeable ‘Rattle’ and though a rattle, contributed much to the harmony of the evening.
1835		Clare Jrnl 27 Apr. 1/5: This agreeable rattler seems to be entirely abandoned by his characteristic and unique vocabulary.
1844		A. Smith Adventures of Mr Ledbury II 21: Mr. Roderick Doo, who appeared to be what the ladies term ‘an agreeable rattle’.
1858		Cheltenham Chron. 27 July 8/3: The Agreeable Rattle. Alfred Chenery is an example to be quoted. He is six-and-twenty, slight and tolwerably good-looking [...] he will never marry; but when his ‘agreeable rattle’ days are over, will sink into a gurrulous [sic] [...] old gentleman.
1866		Belfast Morn. News 20 July 3/5: Disraeli chaffs him unmercifully, and [...] spoke of his noble friend as an ‘agreeable Rattle’.
1873		Edinburgh Eve. News 4 July 2/6: To be an Agreeable Rattler is no longer a qualification for a shop assistant.
1880		Bulletin (Sydney) 3 Apr. 4/2: Cain looked rather darkly on the agreeable rattle by him, and said rather darkly, ‘Change the subject’.
1893		Nottingham Eve. Post 7 Apr. 3/2: Mr Fuller Mellish, though talking somewhat the agreeable rattle [...] played with no small spirit.
1908		Sheffield Indep. 4 Mar. 3/3: Mr Humphreys manages to range [...] over these themes in the spirit of an ‘agreeable rattler’.
1909		(ref. to 1840s–50s) J. Ware Passing Eng. of the Victorian Era 4/1: Agreeable Rattle (Soc., ab. 1840). A chattering young man. The genus has long since disappeared. The A. R. went out with the great Exhibition of 1857.
1931		Northern Whig (Belfast) 6 June 11/7: A Beverley Nicholls Omnibus [...] Mr Nicholls is the ‘agreeable rattle’ of our time.