OK, the scenario:
Central heating seems to work fine, but there is air (a lot - about 1/2 rad) in 3 of the upstairs radiators. I can bleed most of it out. So, fine?
No! The next morning when the system comes on, there is the most almighty gurgling in the system waking up the entire house for a good 30 minutes. The gurgling is from the radiators affected, but sounds like it may be in the loft too (where the expansion tank is).
After a few days of leaving the system alone, it settles down and is quiet.
If I bleed the rads again, I get the morning noise problem again.

Any advice?

Do you bleed it with the pump off?
Has the expansion tank got water in it?
Have you bled all your rads and the boiler itself?
Is the expansion tank definately higher in level than the top of your "problem" rads?
Have you checked if water is leaking from the system by switching off the expansion supply and noting the level? CAUTION Don't let it run dry if system is on! Keep an eye on it.

HTH

Steve W

As Steve says...Bleed the rads with the system off as bleeding rads with the pump on can draw in air to the system.

Check the pump, you can bleed air from the pump also by turning the centre screw, also make sure that the pump speed isn't set to high, you can draw in vast amounts of air by having the pump turned up to high.

It does sound as if the ball-valve in the F&E isn't working as efficiently as it should be, either check the level of your F&E tank and make sure the water level is approx 2"'s below the overflow or change the ball-valve for a nice equilibrium ball-valve (works very well on F&E tanks)

Check for any minor leaks around radiator valves, look for scale build up, if there is excessive amounts of scale then there is almost certainly a slow leak and again, this could be drawing air into the system.

I personally dislike F&E tanks and would opt to change it to a sealed system, not that expensive either!

We had the same problem in an older house.

The ball valve had been tiedup so there was no water in the expansion tank, it kept sucking in air !