Peugeot 207 engine

when the engine is running a vacuum is occurring inside the engine. this is causing a squeak from the right hand side of the engine, something like a bearing or fan belt squeal. you cannot undo & remove the oil filler cap because the vacuum caused is holding the filler cap down. if you turn off the engine, remove the filler cap then start the engine the noise disappears from the right hand side of the engine & you can hear a sucking sound from where the oil filler cap is missing. if you then place the oil filler cap over where it should be fitted the vacuum pulls the oil filler cap down into place & holds it in that position so the vacuum caused is considerable. the engine runs fine & doesnt appear that the vacuum is effecting the smooth running of the engine. can anyone shed any light on this problem - thanks