You can generally hear big ends on start up before you have full oil pressure like you have said and you then dont really hear them (Unless there is no oil pressure!!) until about 2.5k upwards where you will get the rattling sound as a bit of load goes on them.

Again if youve just changed the Followers and not the cam at the same time then sometimes the cam will scrub the bottom out of the follower and cause a tapping sound but this will usually generate a misfire aswell.

More than likely if its a CVH then it will be the big ends but you never know!

Hope that helps - im sure if you mentioned a bit more about it then some more techinical guys might be able to help you to diagnose it