Hi my name is George and I 'm living in Athens(Greece)
For over a year now I own a Ford Sierra Sapphire Cosworth (RWD) with YBG Engine (Green Top).
I'm using MOTUL V300 15/50 Full Synth oil and I 'm having a little problem. when the car is cold it sound like pinto(not so much) until 1800 rpm.
when it's getting hotter the sound stops and after a while i can hear it again but lower not like the cold start. If I accelerate that car when I have that noise then the car is having better idle and the sound stops again.

My engine mods are:
1. Garett T034B
2. Free Flow RAM-AIR filter
3. 4x4 intercooler
4. Grey map
5. Grey injectors
6. chip
7. 044 Fuel pump
8. de-cut free flow exhaust

I 'm using only BP Ultimate 100 and turbo pressure up to 1.2 bar.
This is my daily car (i'm using it to go to work)



P.S. I've tried 5-40 MOTUL oil once and my engine was so silent that I couldn't believe it! but here in Greece the temps are High thats why I.m using MOTUL V300 15/50 Full Synth oil. (I change oil every 2500km with oil filter (Motorcraft))

 

check valve timing, but it sounds like you need new hydraulic tappets

 

without hearing or seeing the car i would probably say needs tappets (hydraulic lifters)
easily bought of likes of ebay ,burton etc
just make sure you buy genuine ones ,these have ina stamped on them if i remember rightly

 

hahahah beat me to it jtech lol