I have an Ideal elan 2 boiler this was in the house when we bought it?

I have an Ideal elan 2 boiler this was in the house when we bought it and has worked alright and is still heating the radiators (although the water pressure on the hot tap is pathetic) anyway about 3 weeks ago it started making a strange noise when starting up..it sounds like there isn't enough water in the system if that makes sense..any ideas what the problem is?