Righto chaps, I've got the same problem...
it started with a heater tap blowing off, replaced that. Discovered a radiator leak, tried all options before eventual replacement. At the same time I got the radiator put in, got the thermostat replaced.
immediately after getting it home, same problem as you have described, air rushing through the system as if boiling, though temp is ok.
Eventually, (month or two later) replaced water pump, flushed head radiator and hoses. Thought I had fixed the problem, as I refilled the levels and bled system properly (have become quite proficient with this task) and heater ran well, car operated at what I thought was close to normal op tems.
very next morning, same blowbac through radiator! Grrrrrrrrrrr
Now the only thing I can think it might be are these three options:
thermostat not functioning properly and causing pressure above the head via steam, which is release when water pump is not functioning;
radiator cap is old and could have lost it's sealing abilities ( these are high pressure radiator systems)
or my head gasket is stuffed, though I think less likely, as no water in oil and no real identifiable steam in exhaust.
For me, I'll try the cap and t stat 1st, much cheaper than trying the head gasket.
It is very frustrating though, but this is the end result of not maintaining my cooling system properly for an extended time.
One other thing I'm curious about, does your air con still work. as the air con and radiator are linked perhaps they may be related? thats a long shot though, and I'd rather be wrong...
juz