S80 coolant bubbling. Possible leak

[Darrellmorgan13]

Hi, I'm new to the site but it looks great!
Recently purchased a s80 t6 99.
Drove the car around 15 minutes on a warm day, up a couple of hills to mates house. When I arrived the coolant reservoir was bubbling, I was then informed that I had a blown head gasket! I had a combustion test done and it came back clean, thankfully! But now I am wondering why it was boiling? I'm not sure if the coolant level has dropped at all, it may have slightly in the last week, but I haven't noticed any drops under the engine.
Just went outside and ran the car for a few minutes, certainly wasn't bubbling, must've been the hill that pushed it over the edge. Thinking back I do recall a couple of previous occasions after parking that I've heard some bubbles but never thought to check.
The radiator cap looks good. The fan came on when I started the car. I had the car serviced last week and they replaced the old coolant expansion tank as it was leaking and resealed the thermostat in it.
Any advice on what the issues could be would be hugely appreciated!
Thanks in advance
*edit: After a 20 minute drive I returned home and could hear the bubbling noise again, it continued for a few minutes after the car was parked. The fan was not on, I started the engine to see if the fan would start and it did but was blowing very hot air into the engine. If I try to remove the expansion tank cap the coolant level rises even more and increases the bubbles.