What can cause my combi system to lose pressure?

I recently had my combi system flushed and for a week it held pressure, and was, as in the past, quiet. However, after the week, it gradually lost pressure, which happened faster and faster as time went on. I bought some x400, put it in for a few days, flushed it out, and added x100. For a couple of days it was fine again, but now it has started to play up again. Also, the boiler makes a very loud humming noise which vibrates through my house. The guys who did the power flush sid the pump was on the way out, could this be the cause of all my problems?

If I don't use the heating, but only use the hot water system, nothing happens except the noise on a reduced level. I am at the end of my teather as I have sold my house and now this happens.

The boiler make is Biasi 24s.

If anyone can help me I would be so thankful!!!

P.S. Anyone with info', I would appreciate a reply to jamie.missenden@virgin.net.