Last three mornings just after I start it up I hear a clicking-clacking sound that lasts for only a couple of minutes and then goes away.

It almost sounds to me like the cam chain is fetching up some how but I am not at all sure this is what I'm hearing. Could it be the vacuum pump, it is a recent replacement.

The sound is quite loud and almost sounds like whatever it is, is getting ready to blow, but then it goes away!

Anyone think they might know what this is?