A centrifugal fan 168 kW, 1480 RPM Overhung with Flexible pin bush coupling was running normal with vibration max 3 mm/s since 4 years. There was a sudden increase in vibration at 2x RPM frequency up to 8 mm/s inin coupled condition at motor NDE side Horizontal and fan NDE vibration observed max 3.6 mm/s with minor looseness symptoms . Motor Solo run was taken and observed with 1x Line frequency(49.97 Hz) observed dominant in spectrum with vibration observed up to 3.4 mm/s in motor NDE horizontal. Motor was observed with humming sound in solo run. During the coast down Water Fall spectrum data was taken and found that the line frequency disappeared immediately after power cut-off. what could be the possible source of excitation of 1x Line frequency?