Mini Cooper S '07 Clicking noise

I'm considering buying a manual 2007 Mini Cooper S so me and my
boyfriends brother(who's licensed to work on cars, works at a dealership and
has been trained to appraise cars) went to test drive it and there was a little
clicking noise when it was in idle. We drove it and took it on the freeway and as
soon as we hit 60mph the check engine light came on. It still ran fine and it
didn't feel like anything was wrong, no power loss or anything. When we
parked, you could hear the clicking noise more clearly but it calmed down after
a little bit. He said he's not sure what it is, but he thinks it might be an exhaust
valve. I'm planning on taking it to a shop to have it checked before I buy it and
see what's wrong with it but I just wanted to be a little informed before I go in.
Any one else have a problem like this or know what it might be?