I had something similar - but tapping noise (passenger front side too) would only appear when the engine was getting hot. Root caused it to a loose centre upper idler (shown below
)
.
Was causing the cambelt to flap and give off a serious smell of melting rubber too. Anyhow - fixed it by replacing the tensioners, belt etc. I think I was really close to having the engine written off with that one. By the way my cambelt wasn't due to be changed for another 13K so even the 40k cushion doesn't always protect you
Never assume!!