TOKYO, Feb 18 (Reuters) - The Bank of Japan may replace some numerical guidelines for its purchases of exchange-traded funds (ETF) with a pledge to ramp up buying when markets become volatile, three sources familiar with its thinking said. The move would allow the BOJ to more flexibly slow purchases when markets are stable, while reassuring investors it will step in forcefully when shock events trigger turbulence, they said. While there is no consensus on the final decision, the idea is among options being floated within the BOJ ahead of a review of its policy tools due in March, the sources told Reuters. The BOJ will conduct the review at next month’s policy meeting to make its massive stimulus programme “more sustainable and effective,” as the COVID-19 pandemic heightens prospects of a prolonged struggle to reflate the economy. The BOJ has been buying ETFs for a decade as part of efforts to pull Japan out of deflation. It now pledges to buy ETFs at an annual pace of roughly 6 trillion yen ($56.7 billion) and by up to 12 trillion yen, a commitment that forces it to keep buying even when stock prices are booming.