Mumbai: The benchmark BSE Sensex witnessed volatility after its three session gain, though managed to recover most of its losses to end marginally lower by 43 points to 28,420.12. Earlier the key indices opened positive, but soon wavered on caution ahead monsoon session of Parliament today, which is expected to stormy in the midst of number bills to be passed including the key GST bill. “Investors seem to await the outcome of Parliament’s monsoon session and corporate earnings with big names like HDFC Bank, Infosys, Hindustan Unilever results scheduled tomorrow,” said Gaurav Jain, Director, Hem Securities. Some key Bills such as land acquisition and goods and services tax (GST) are expected to be presented in Parliament. Tata Motors lost big-time, down 1.89 per cent.