Noise like hitting a tin can with a stick?
 

My 2000 Boxster 5-speed has made a strange noise on start up a few times. I can only describe it as in the title of the thread, like hitting a tin can with a stick. This happens only after the battery has been disconnected and reconnected. The car starts with the noise and a very rough idle... once I coax the engine speed up to 1500 or 2000 rpm it starts to run smoothly and after a minute it can return to a normal idle. After that it starts and runs normally.
The only thing that I can think that would cause the noise is detonation in the exhaust system. Could this be caused by the engine control losing its settings and needing to reset itself after loosing battery power?

Yes, sounds like what you mentioned. You might have a vacuum leak. Any codes?

I've had P0450 and P0455 (both are to do with the evaporative emission system) pop up, cleared them and they did not come back.