Hi all my friends 2001 golf tdi ALH 90hp has a bad knocking noise sounds like a Transit.

on tick over all sounds good but if you give it a quick tap on accelerator it make a loud knocking sound like it's coming from back of engine around the manifold area, if you slow accelerate nothing sounds normal, if you push the clutch in and tap the accelerator the knocking is even louder.
ive checked cam area ect. All looks good.
no exhaust leaks

cant seem to work it out right now. Tomorrow I'm having sump off to check bottom end areas as starting to think big ends or small end bearings going, or maybe an injector knock not to sure.
any help will be considered.

thanks Paul.