Radiators?

New Potterton Promax SL 15 1yr old supply and fit by British Gas, its recently had replacement heat exchanger fitted by Baxi under extra 3months warranty.
As water initialy circulates through rads on fire up the water in some rads 'Gurgle' does this mean 'sludge'? no air only water when rad air valves open and on the pump pipe is auto air vent so assume air is displaced through this valve. What causes 'Gurgling' other than air in the system.
I will say thank you to replies now in anticipation of them.