Cooling system pressure leak?

I just purchased a 2000 Discovery and have noticed air/pressure coming from the radiator reservoir cap when the engine is hot.  I do not seem to be loosing fluid (just noticed the noise a couple of days ago) and the engine doesn't over heat - is this normal?  The used car lot I bought it from says it is but they are not a Land Rover dealer.
                              Thanks for your help.

It would be a good idea to replace the cap, just to make sure there is no problem.
If it continues then a coolant pressure test might be called for.
Let us know how it turns out.

Thanks Disco Mike, you were right on.  I replaced the cap and have been driving it for two days with out any problems.  I really appreciate the help - the dealer had me scared with their suggested head gasket replacement!