My Crown Vic has developed a knock when starting. It happens cold or hot but most often when cold. It'll do it when hot but only if it's been shut down for a while.

It sounds to me like oil starvation of the lifters. I'm using Castrol 5W-30 with a Purolator filter that has the anti-drain back valve.

It just turned 75k and the knock started about 5K ago. The knocking stops after about 3 - 5 seconds. Seems to be just a little behind the oil pressure gage, in other words the gage shows full pressure then a second or two later the knocking stops.

Any suggestions?

I would use a regular oil filter. it does sound like it starves for oil

A "regular oil filter"?

Do you mean a filter without the anti drain back valve or a Ford/Motorcraft filter?

I've been using the same model Purolator filter for about 40K miles with no problems.

Yes one with out a drain back valve.