It's a Ravenheat CSI 85AT. It was self-installed (well... not me actually
I'm passing this along for someone else) about 2 years and has worked fine
since.

The problem is that the boiler pump appears to be making quite a loud
ticking noise (the volume varies) occasionally -- when turning on the hot
tap, or when the central heating comes on. Although the pressure within
the central heating loop is well within the range described in the manual
when the noise is occurring, if he adds more water pressure (pushing it up
to 2 bar) the noise disappears.

Does anyone have an advice on curing the problem. Is the pump
actually the problem and would replacing it solve it?

Thanks in advance.