Poor hot water flow rate on my Vokera Linea Plus combi boiler?

Hi

I would be grateful if anyone could advise what might be wrong with my Vokera Linea Plus Combi boiler. The boiler is approximately 7 years old and has operated with out any problems, it was last serviced on 1/8/07. Over the last few days I have been experiencing poor hot water flow rates (approximately 8.5 ltr/min @ 30 degree temperature rise). At the same time I noticed the boiler started to make a strange noise when hot water was flowing, it sounds a bit like an air & water mix flowing in the pipe between the cold inlet valve and the water flow regulator which is situated on the diverter valve/ flow switch manifold. No fault codes are showing on the display and I don't have any water leaks.

Any advise or direction would be much appreciated.

Thanks Dave Turner