Did the new engine have a new oil pump ?
Check in the first instance your oil pressure, what sort of noise is it as it is not unusuall to be able to hear a GENTLE click click of the tappets, if you can hear it inside the car with the bonnet shut it is defo wrong though, unless you have good hearing!!
Most important of all what tappets did you use ?, were they genuine INA, ie FORD ones, if they were PATTERN you have your answer, though even they should last slightly longer than this.
I would as it is a new engine use a slightly thinnner oil like SILKOLENE PRO S 15/50, and genuine ford filter.
tabetha