changing rocker arms.How do I remove compresser.To take valve cover off.

You have an oil pressure problem. I hope you are up to removing the oil pan and changing the oil pump and pickup screen. If you need help contact me at autoknowhow@gmail.com

Is it a deep knock or a higher tapping sound

Rocker arms make a rapid ticking noise when loose or defective! Also usually more noisy when cold. If you hear a dull, slower knocking noise, don't think it's the rockers. A mechanic can tell you what it is or is not, so your best bet is let one listen to lt. Save yourself all that work in the process! Pulling the V/C's is a job you may not need to go through, especially on a van!