With you saying the noise goes when the bleed bolt is undone then that means something is blocking the flow and with the bolt open that to me is the water finally finding a route to get out and thus circulating the water through via the pump out the bleed bolt. I would as Mat suggested have a look at the radiator and if so remove it and give it a good blow through and clean out. that sounds like the problem.

With regards to the cap having a vent. I think, (not that I have looked at one really but), the standard yellow cap doesn't look like it has a vent, I think thought the rubber seal in it will give if the temp gets too high???? were as one like mine which is a spec r header tank, its cap does have a vent flap valve thingy in it.

Lets us know how you get on bud.