Whistling noise coming from my vauxhall astra?

To start with iam a qualified mechanic so i should know what im talking about. I own a 04 plate Astra with a loud whistling noise coming from the engine after being run for 20 mins or more. the engine code is XEP (the water pump Does Not run off the timing belt). If the main breather is disconnected from the back of the rocker cover the noise stops immediately. The cause of the noise is excessive negative pressure in the rocker cover and crankcase. Another way to check is to remove oil filler cap and if excessive vacuum is felt then the only course of action is to replace the entire rocker cover assembly. Rough cost - £115 + vat Inside the rocker cover is a plastic valve and diaphragm that is the cause of the noise. Unfortunately this can not be replaced independantly. Hope this is of some help