Gurgling noise after a new Radiator was fitted

Greetings all. I had an engineer around yesterday who fitted a new Radiator. We now have a continuous gurgling noise. We are using a Boiler Mate 2. The Engineer did bleed the new radiator. Any help or advice would be very appreciated + Happy new year all "Are you sick of hearing that one yet" :-)

I found this on the web
Faults known to occur in the Boilermate II:

1) Depleted water in the thermal store.

BoilerMates have a filling, or top-up tank fitted to the top of them, which is there to fill the unit with water. This may or may not have a float valve connected to the mains water supply to fill it. When there is NO mains connection, there is usually provision for manual filling by the user by means of a tap on the wall nearby. When the user doesn't realise this, water lost from the thermal store through evaporation and/or leaks can prevent the unit from working. If the water level falls too low, the heat exchanger coil ceases to be immersed in stored hot water so when a hot tap is turned on, the unit will not deliver hot water. The answer is to check the water level in the top-up tank and top it up to the waterline embossed into the wall of the cistern. This problem is usually accompanied by a noise of water trickling or flowing inside the unit. This is caused by the flow from the boiler discharging into the unit above the water level inside and making the 'pouring' noise. This can be very bad for the system as accelerated corrosion usually results from the constant aeration/oxygenation of the circulating water.