It may be that the circulation pump is worn or on it's way out. It may be worth stripping it down and refurbising it.
http://www.alde.co.uk/downloads/alde_2737-62-63_service.pdfAnother thing may be worth checking the pipe work is not rubbing anywhere.
Can you lift the seats and narrow down where the noise is coming from?
Or do you just mean that its running ok but the general noise is the issue? Like the fan on a blown air heater