The Economic Noise Index is designed to gauge background economic noise and expresses as either more positive noise or more negative noise. It incorporates 2 components: the Citi Economic Surprise Indexes, and the Economic Policy Uncertainty Indexes.
Basically a higher reading for this index indicates more positive noise: i.e. economic data is generally beating expectations and surprising to the upside, and economic policy uncertainty is generally lower than usual.
Lately economic data has been missing against arguably over-optimistic expectations and policy uncertainty has been elevated in the Trump era and as the Fed moves towards monetary policy normalization. Clearly the worse these factors become and the more persistent they stay bad the greater the likelihood is of a market accident, for example the almost mythical 5% correction.