changoo

Hi. I have a 2003 Audi A3 2.0L TDI, the 140 bhp model, and I’m loosing coolant somewhere!!
I brought the car in February based on the model and the engine as they run "forever" but loosing coolant somewhere . I've taken this too a garage and they have pressure tested the engine, and there are no external leaks.
They said "there are tiny bubbles in the expansion bottle which could mean there is a crack in the head and the pistons are moving so vigorously they are pushing the bubbles up in the system and into the expansion bottle".

Looking for advise; does this sound right, and is there anything I should be checking first before the possibility of spending an absolute fortune on parts and labour?!?! ?! HELP!