Okay so maybe it isn't so much screeching as an intermittent background noise like metal on metal. Machine is a 1996 Bridgeport VMC 600/22 and has been a great machine since we got it last year, milling lots of aluminium. However last week it began making the noise every 20 or 30 seconds for a couple of seconds at a time. It's loudest at 6,000rpm. No other noise, rumble or vibrations. Any suggestions?