Ford Transit noisy engine?

Hello i have a 2004 85t260 and was wondering if anyboby can shed some light on my problem.The van has done 53000 miles and has become very noisy nearly deafining.When you start it all is fine but as soon as you touch the accelerator it still sound like a transit but is about 5 times louder when i lift the bonnet and rev it lightly its still deafining but kind of knocky aswell.It drives fine still v noisy up the gears but take foot of accelerator back to silence.Hope someone can shed some light on this.Regards