Morgan Stanley seem to have kicked off the current round of warnings:

Here in local media (Australia) the Australian Financial Review has done a bit of a round-up of alarm bell calls:

Bank of America

  • forecast a 6% drop in the US benchmark index to 4250 by the end of this year
  • predicting a soft 2 per cent rise to 4600 in 2022.

Citigroup

  • any minor correction is at risk of being amplified given the extent of bullish positions

Goldman Sachs

  • While the broad US market outlook is solid in our central case, we think peak cyclical optimism in the US may be behind us

Credit Suisse

  • retaining its underweight recommendation based on regulatory risks and extreme valuations

There is a bit more info at that link to the AFR above.