ok so i recently bought an Acura EL with a forged D16 with ported and polished head, eagle rods, wiseco pistons aswell as ACL racing bearings and ARP head studs. The problem is, ive noticed it has a little knock or rattle coming from the exhaust manifold when at around 2000rpm. I dont know if this is normal or not, any thoughts?

here's a video
https://www.youtube.com/watch?v=WjcKpZKGDUE&feature=youtu.be