Noisy Fabia Comfort Estate 1.9TDi

Hi everyone.

My Fabia Comfort 1.9TDi estate has been getting progressively noisier over the last week. I first noticed the low pitch 'buzz' at 20-25 mph, and then heard a second higher pitch tone seemingly coming from the rear left hand side at 45-50 mph. The handling seems fine so I'm not inclined to think it's steering or suspension related, and the engine sounds ok at idle so I'm at a loss as to what to do. I took it to a workshop on Monday, they said they couldn't find anything obvious that would cause the noises. Could it be exhaust related?