What does this mean?

[George Thompson]

Here is a story about skulduggery in NYC government (yes, even in 1830).  It contains what seems like a proverb or quotation that I can't interpret, except in the obvious general sense "there's mischief afoot"; but why "mischief under the white heap yonder."?

1830:   But again delay after delay was interposed, until at length an entire new scheme was matured and brought forward in the common council.  ***  It now began to look as though there was "mischief under the white heap yonder."
         Commercial Advertiser, July 20, 1830, p. 2, col. 2  [one sentence summary of the scheme omitted]

GAT

George A. Thompson
Author of A Documentary History of "The African Theatre", Northwestern Univ. Pr., 1998.