Potterton Profile noises?

I have a Potterton Profile 60e which is about 16 years old and still works fine, the only part that has been replaced is the fan.
Recently it has developed some odd noises and I would be interested to know if this indicates a potential problem.
On start up after a few minutes it emits a fairly hefty clunk and just generally creaks and groans a bit throughout the heating cycle, I can't remember it doing this when it was new, does anyone have any idea what could be causing this?

Derek
December 2009