"He has been a great investor for our company, stabilized our company," Moynihan said about Buffett during an investor conference in New York. "I don't know what exactly he is doing because frankly we can't ask."
Buffett's Berkshire Hathaway (BRKa.N), the biggest shareholder in BofA, has been trimming its stake in the U.S. lender, making total share sales since mid-July of nearly $7 billion.
Berkshire holds a stake in Bank of America of about 11.1%, according to LSEG data.
Moynihan said the market is absorbing Buffett's stock sale.
He also spoke about the proposed changes in capital rules after the Federal Reserve's regulatory chief on Tuesday outlined a plan to raise big bank capital by 9%, significantly easing an earlier proposal after intense Wall Street opposition.
Moynihan said the proposed changes are manageable for Bank of America, and it will continue to buy back stock, but cautioned that higher capital requirements for large banks could reduce lending.
"If our capital goes up by 10%, it stops us from making $160 billion (in) loans we would otherwise make. Those loans would go to small businesses and middle-market companies at competitive rates," he said.
Speaking about performance in the third quarter, he said investment banking revenue could be around $1.2 billion, little changed from $1.18 billion in the same quarter a year earlier.
Moynihan said middle-market investment banking - transactions below $1 billion - has been growing at a strong double-digit pace.
He said trading revenues will grow at a low-single-digit pace in the third quarter.
Rival JPMorgan Chase (JPM.N) said trading revenue is expected to be flat or rise 2% in the third quarter, while Goldman Sachs' (GS.N) CEO on Monday said the bank's trading revenue will probably slip 10% in the quarter because of sluggish conditions last month.
