Another Noisey Boiler!

Seems a very common subject! Can someone point me in the right direction please, I have a 7 year old Potterton Suprima 40 boiler. I have just started experiencing the howling noises, which seems to be eminating from the boiler. It happens if either HW or CH is on or both together. I presume its traped wind (fa-narh), I have bled the hot water circuit (valve near the water cylinder) and all the rads. The pump is nice and quite so no air there. I believe its the boiler as the pipes only make a noise when its lite, and they die away as soon as the boiler shuts off. Also there's plenty of water in the tank in the loft. Does it sound like i need to get it flushed out because of a blockage which might be starving the boiler of water? Is theer any other way of bleeding the air out of the system?
Please help - its waking the missus up in the morning.........

Ta in advance.

flushing is a good idea the boiler is kettling (boiling in spots)
would think sludge try fernox system restorer
comes in a cartridge and goes in a rad
once had an ariston screaming its guts out
dosed it and within an hour
sound of silence