Noise from Radiator - Advice please

Hi

Looking for some advice please as we started getting a water trickling / gurgling noise from one of the radiators and after bleeding the system a few times (not much air came out) we thought it may be best to drain and add inhibitor as it had been 5 years since last time that was done.

We have an Ideal condenser boiler with a Grundfos Alpha 2l pump, I think the system is called open vented as has small tank in loft that fills the system and has an overflow pipe going back into the small tank?

We drained it then added Sentinel x400 when refilling and left that for 2 weeks using the heating as normal so it has been on for about 7 hours a day then drained again and refilled and added Sentinel x100.

The noise still happened and we put this down to air being in the system so continued to try to bleed the system.

As the noise continued after days of bleeding the system, we looked at the settings on our grundofs alpha 2l pump to see if that is something that could be causing it although this has remained unchanged for 5 years. When we changed the settings on the alpha 2l pump from what it has been on the last few years from speed II.

We tried the PP2 setting on the pump and it started off fine and was ok whist the heating was on for over 2 hours. Then after the heating had been off and then came on later in the evening it went worse with it sounding like a car misfiring with gurgling and whooshing sounds of water sounding like its coming from all over the house.

So we are at a loss as the sounds in the radiator are still there and sounds like water going down a drain.

Is there anything anyone can suggest as It sounds like air is getting into the system somehow but not sure how to even begin to locate where it would be coming from?

What I did notice is that the small tank of water became dirty within the two weeks since draining and refilling, so seems like water is coming back up the vent pipe at some point, not sure if thats also normal but thought I would mention that?

Any help would be appreciated as not sure where to go with it now?