Bubbling expansion tank
 

2001 B5 1.8. Noticed that there is some sort of bubbling occurring at the top hose in the coolant expansion tank. Just had the coolant flange replaced (thinking of going aluminum next time if it fails again). Thought that might fixed it but it didn't. Any idea as to what the reason is?

If the coolant is constantly bubbling then something is pushing gas under pressure into the coolant. You are not going to like it, but the most likely is the head gasket. I would go get a leak down tester and pressurize the cylinders and look for the bubbling when each cylinder is under pressure. If it does you have verified the problem and can go from there.

That's what my dad was thinking. I recently had my timing belt done (teeth broke = bent valves) so I had to get the head remachined. I wanted the shop to double check any cracks in the head but the owner is very stubborn into saying that it's not the head (should've never taken it to him). I'll try bleeding the system and try the leak down tester if the bleed doesn't solve it.