9N Polo 1.4 Engine noise.

My car is a 2003 Polo 1.4 S (16 valve BBY engine) - 29,000 miles on the clock. I've always had it regularly serviced from new, but now find the engine has developed a light knocking/rumbling noise, most noticable when changing gear as the revs momentarily drop away but also (less) noticable when revving engine from idle and listening as the revs fall. There are no problems with the running of the car, tickover is smooth with very slight belt noise, still pulls well and never any hiccups. The car is never driven hard, the engine very rarely exceeding 3000 rpm. Maybe the engine is just starting to show signs of wear?
Has anyone else had a similar problem?