Gargling noise from vertical radiator

After the heating had not been used for a few months, finally turned it on a couple weeks back. One of the rads makes a gargling / water whooshing sound when the heating is switched on, turned off or when someone turns on a hot water tap (diverting from heating to hot water taps).

tried bleeding the rad and waiting about 30 secs of water coming out but still the same.

It’s a combi boiler and the rad in question is a top floor vertical radiator.
Boiler was fitted a year ago and system was flushed.

Any idea what could be causing this and how to fix?

Any help on this? Driving me mad. Had a Few plumbers come out who couldn’t figure it out. The rad is only partially hot so seems like there’s air in it but after bleeding for a few mins it only heats up for a few days and then back to partially hot.
Also put leak sealer in system thinking it may be a microscopic leak but no change.

Well unfortunately IMO your first mistake was putting leak sealer into it. It's nasty stuff and I don't think it should have been invented.

Vertical column/bar radiators are notorious for acting up, being noisy and not heating up correctly, especially if it is the highest rad in the system. After a few days and when it starts to act up, can you bleed air out of it or is it still just water?

If no air then it's probably a circulation/balancing issue, what size of pipework do the rads have. Was the system balanced properly initially?

No air, just water comes out. not sure on piping size but looks standard, 12mm?
The system was balanced by two different engineers at different times. Most rads, if not all were set to about 1/4 to 1/2 turn.

Is it upside down?

No. The baffle inside the rad is at the bottom.

creative101 said:

Most rads, if not all were set to about 1/4 to 1/2 turn.

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IMO the rads further away will require a higher flow to allow the hot system water to radiate evenly, that's to ensure all the rads will warm up at the same time, especially if the rads are over more than one level. A large vertical (column?) rad, especially if it's at the top of a system, will also need more flow simply down to it's design requirements.

Strictly speaking balancing is to ensure that the flow through a given radiator is correct to ensure the correct drop across the rad and that should ensure that the rads is receiving the correct flow, this doesn't always ensure that ll the rads heat up at the same time.
It is just as important to ensure that the rads receive the hot system water at the same time, at a balanced rate, across the whole system. This will ensure that they all heat up at the same time, ensuring the house warms up at the same rate. This is especially important with larger/longer systems over a number of floors. So balancing, IMO, is a 2 part process.