Could be many things. First, I would be reluctant to use an engine flush since you could cause damage to engine bearings if the flush thins the oil to much. I started using diesel motor oil and now use some atf as per rrtoadhall to assist in cleaning the engine. None of this has reduced our engine clicking but it has not gotten worse, either. Our problem, I believe is that the steel inserts in the rockers work loose and this creates the clicking noise we hear. Our rangie falls within the vin series where this can happen, so search for that and see if you are close to ours which is near the end of the series. Another possibility unfortunately is a bad cam lobe or 2 [or hydraulic lifter]which would create the excess clearance. Low oil pressure due to plugged screen, poor pump or worn bearings is also possible. Do you have any history on the vehicle which would tell you what someone else may have done [ie something wrong] Slick 50, Lucas oil stabilizer [which we use] helps to thicken oil and may just solve your issue or a higher viscosity oil may do it. The lifters are designed to take up about 20-30 thousands clearance away ]to reduce noise]yet still provide adequate valve seat pressure for heat transfer so the valve won't burn. This clearance is set on assembly of the valve train system. good luck and let us know if you find your solution.