As the Goldman strategist writes, even as corporate defaults remain near historically low levels, froth (there's that word again) "has been building in the form of corporate leverage. While this may not present a near-term risk, the widespread increase in debt resulting in stretched leverage metrics bears watching, in our opinion."

Goldman adds that while the pullback in Energy earnings in recent years has stressed aggregated Net Debt/EBITDA, even excluding that sector, the ratio is at the highest point since the financial crisis.

Finally, Boroujerdi makes an interesting caveat: "Given we are 8+ years into an economic expansion, we believe it's prudent to also view this via a "normalized EBITDA" lens (i.e., median NTM 2007Q1-2017Q1). On this basis, aggregate leverage (ex- Energy) would move up to 2.1x, roughly 20% higher than current levels and 18% above the prior cycle peak."