coolant bubbling in the expansion tank

Hi folks can you help?

I just bought a 2010 C3 Picasso Diesel Exclusive and after a week I heard bubbling over the radio. No overheating, but the heaters stopped working. Have flushed out the coolant system which was dirty. Pressure was severe when driven and was told maybe Head Gasket but no oil in the tank and the oil cap clean and no smells. Bled the system of air when done, the front and back nipples. Have done this twice now flushing out and last time with water, now it is getting blacker still no oil. While idling there is a constant rhythmic air bubbling in the expansion tank with cap on and off. All pipes to radiator are hot, well were now the top is and the bottom one isn't as hard.

I hope it's not really bad but any help from the experts here would be great and much appreciated, so I know what to get done. I love this car and do not want to give up on her.

DR

Hi

Before you do anything else, do the diesel head gasket test.

A positive result from this can save a lot of wasted time and money.

Thanks so much mate I will do that, Have a great weekend