311cdi Flat spot and a squeak

[red307cc]

I have a 2008 311 which has developed a fault which has me slightly baffled. If I try to accelerate even moderately heavily when going up the gears I get a flat spot which is accompanied by a noise from under the bonnet which sounds a bit like a small dog yelping. If I ease off the accelerator everything is fine. Over the last day or two the issue is getting worse and I'm now losing power on inclines with a constant squeak squeak squeak