I have a 2002 E350 with Quigley 4x4 conversion. Nice van but I have developed an engine knock. To me it sounds like a rod knock. It started out being at engine idle only and only when it was cold. Now it gets louder as the engine rpm's increase. Here is the confusing part. It doesn't do it all the time.

Sometimes I can't hear it at all.

Sometimes it only does it at idle.

Sometimes it does it in the morning and not later in the day.

Sometimes I can hear it as I drive down the road. The engine drinks a little oil but never see any smoke and cooling system is fine. Just under 100K miles with regular oil changes.

I hope this is the right place to get some help because I am not sure why it could come and go like it does.