Air in system

[John1954]

Hi, I have a combi boiler Vaillant. I have recently fitted the Fernox TF1 filter,and thourouly cleaned each rad after having fernox cleaner in the system for over a week. Ever since the rad in the hallway constantly gurgles.I have tried all sorts of bleeding options to clear the problem including turning off all the rads and bleeding each one in turn, bleeding downstairs first bleeding upstairs first. I have hosed the offending rad.Is there something I am missing , is there air trapped in the boiler, if so how do you get rid of it. Could it be down to the filter I have fitted. I am at a total loss what to do next. I have bled it now six times but still no good. All other rads are fine and all get hot even the offending rad. All I can think is I have got a bad rad, but it was fine until I started all the cleaning of the system.