Hi Martin your problem could be a number of things,the fact you have ok preasure in the other taps rules out too much preasure from your combi,I take it the tap in question is the bath tap,this tap by tradition is fed by a 22mm pipe unlike the others which have 15mm,this was done so that your bath filled up quicker from your hotwater tank which in most cases is nearly the same height as the tap on the bath so more volume was needed hence 22mm pipe,of course you have a combi boiler now but the pipework will not have been changed I suspect,so you will get more pressure out of that tap,possible the best cure will be to have new taps put on ,from your question the tap is making a noise so it is worn,the average life of a bath tap is about 15years before they need some help,you also say you sometimes do not get any water again it sounds like the bobit inside the tap is worn out,if you are not good with plumbing get somebody to change it for you.
Best Regards