Land Rover Repair: vacuum moan, vacuum leaks, vacuum leak

Question
2003 Range rover has a short moan from behind the dash at highway speeds. I tried to time it but it is irregular and sometimes occurs within a minute and at other times takes a few minutes. I think it sounds like a vacuum leak or is related to the vacuum system. Any ideas where to start? I like to have a real good diagnosis before I take it to the dealer so they don't waste too much time. Thanks.

Answer
Hi Les,

this is a new one for me. How does your Rangie perform?  Do you notice a power loss? Does it happen only with your cruise control on?  It is more noticeable on long uphills?  It would stand to reason with vacuum leaks that engine performance would suffer.  Same goes with cruise control.  

Regards,
JohnMc