Hi I have a volvo fh13 460 2010 which seems to be over pressurising the water header tank and can be heard whistling through the cap, any ideas would be appreciated.
Check the compressor head. If that’s OK replace the injector sleeves.
Thanks for that, thought it might be related to the compressor.
Head gasket. 
warcake:
Check the compressor head. If that’s OK replace the injector sleeves.
Completely agree with this, both are common problems with this engine at 6-7 years old. It’s more likely to be the injector sleeves and you can check by shining a torch into the fuel tank and seeing if tiny bubbles are coming up from the bottom of the stack pipe when the engine is running, if there are then it’s the injector sleeves.