Bubblegum

Hi I wonder if anyone can give me a little advise. I've developed a tapping noise in the front cylinder. Engine is quiet as anything when I start it up but after about five mins of running ie when the engine is getting hot it begins. Literally it sounds like chang chang chang like metal on metal when I excellerate it disappears and as the engine slows it returns. When turned off and engine has cooled and I start it again it's quiet. Any idea's what this could be. Thanks Bubblegum P.s I'm running a Virago 535