noisy start up - open vented system - air in pump, etc.

Hi,newbie here. My problems have been ongoing for a while now. Not sure when they started, but have recently drained the system to refit a new radiator. Did that and refilled system. Bled all radiators periodically as air was getting trapped at the top of the upstairs rads.

I continually get what I think is air going through the pump and up the rads. Every 2 minutes or so I hear what can only be described as a sound of mini ballbearings shooting through the feed end of the rad, but I get a constant air and water mix sound from the pump.

I've got trvs fitted to 3 rads. One was loose and when I tightened it that reduced the noise in rad slightly.

When I adjust the lock shield valve to nearly closed and reopen slightly I can hear the flow of water thru the rad quite clearly as if there's alot of pressure flow.

This leads me to think that the flow of water is too fast (but pump is set to minimum of 1 and have altered this to 2 at times, but doesn't quieten the noise of air/water mix).

Is my expansion tank too full or am I thinking in the wrong direction? I heard that the level of water should be low when the system is off and cold, but surely the ballcock would trigger the tank to fill?

Btw, when boiler starts up it makes a gushing noise for a good hour before it quietens down. Rads don't heat up instantly either - takes about an hour before they get warm.

Any suggestions greatly welcomed.

Thanks, Steve

Sorry, an addendum - my boiler is an old thorn olympic (don't know the model number) but it gets serviced every year and the bloke tells me i don't need a new boiler as this one works fine. He recently came and changed the thermocouple as the pilot wasn't lighting.

What would happen if I undid the big centre screw in the pump? I hear you can bleed the pump this way, but I don't know how far to undo the screw.

and pump is a Grundfos Selectric4.

Hope this helps.