Engine Noise When Cold:
A light engine knock, after cold start, that gradually dissipates as the engine warms up and is virtually undetectable (from inside the vehicle) once the engine has reached operating temperature, is a normal characteristic of these engines. Repair attempts to reduce this type noise are generally unsuccessful.
If you have a vehicle in which an engine noise is other than that as described above, be sure to take the time to check all possible causes prior to condemning the internal components of the engine.
Before replacing parts in an attempt to eliminate engine noise, the engine should be inspected externally and internally for another source of noise. A look at the engine oil is a good place to start. New engines will have a small amount of metal particles in them, but after that should be relatively free of metal.
Another area to look at would be the timing belt tensioner and the belt and sprockets. There have been cases where noises under the belt covers and from external components have made noise that sounds like a deep internal knock.
I’ve had good luck with running Rotella T6 in the winter on my Subaru WRX/STI.
Shell Rotella (550019921) T6 5W-40 Full Synthetic, Heavy Duty Diesel Engine Oil (CJ-4) – 1 Gallon