if I run my 03 grand caravan, 3.3. for a good distance, 75 miles or so. I get a high pitched whistle from the rear(sounds like passenger side). it doesn't increase or decrease with motor acceleration or deceleration. It does it sitting at idle also. no louder or softer.
i can hear fuel pump kick on at start, and the whistle has caused no running or starting problems. just annoying

 

Do you have rear A/C ?

 

yes, but compressor sprung a leak 2 years ago, and havent used it since