1999 Vauxhall Astra vauxhall making a terrible noise

I have a 1999 vauxhall astra 1.6 8 valve sohc, engine x16szr, i recently noticed a popping/tapping sound coming from under the bonnet, i sought advice from a friend who suggested that it may be a blow in the manifold, so i removed the covwer but there was nothing there, i have also done an engine flush and i changed the cam followers but it is still no better, the weird thing is it only happens when i accelerate when i idle or if i am maintaining a constant speed no noise, but when i speed up the noise happens, it seems to have baffled everyone i know and i am really stuck. Can you help please.