In my opinion;
- The first thing you should do at least once a year is check your CCV diaphragms (It only takes 10 minutes) This may be the cause of your whistle noise.
- The negative pressure is good and should be felt at the dipstick tube.
- If your diaphragms are working correctly, it has been my experience that when you remove the dipstick or oil filler cap for a short period of time, it will not effect the engine RPMs. (Removing the filler cap while the engine is running makes a mess)
- If you look closely at the CCV caps without the nipples, there is a small hole where the nipple would be. This allows the diaphragms to operate correctly but if you have a bad diaphragm, this could also be the source of the whistle