Hissing overflow unit

I noticed recently that there was a hiss from the overflow on my Freelander 1 1.8 2001 car. There was also some cooling fluid loss. I topped it up but the hiss continued. I tried tightening the top and giving the pipe connectors a tighten but it seemed to continue.

Today there has been no hiss and no cooling loss.

A few months ago the Head Gasket was replaced but the garage suggested I also replace the overflow tank although it looks in good condition.

It also needs new tyres and exhaust for which I have been quoted £111 for each tyre (Pyrelli Scorpions) and £127 for centre and rear exhaust units. The former being £79 but they are suggesting I replace both units as the connection is not usually good if you just replace one unit.

I can't get the emmissions checked with a leaking exhaust so I was going to tackle this before bothering with the tyres due to cost constraints.

Any suggestions appreciated.

Change the expansion tank and cap, they are prone to cracking and the design has been modified to improve them.

Had changed them and in the end also had to change the head gasket - \325 ouch! Second time in 10k miles