Siren or whistling noise

[JamesHinson]

Wednesday my 1998 Navigator started making a siren like or whistling noise. Noticed early in the morning. Would seem to get louder when accelerating. Does not seem to make noise when sitting idle, but will make noise if I reeve the engine. Made same noise later that day. Checked belt, power steering fluid, trans fluid, all okay. Thursday same thing. Called my mechanic who listened to noise over phone and thought it might vacuum leak or pulley. Made same noise Friday, went to mechanic who said it sounded like noise coming from alternator. Made appointment to have replaced on Monday. Saturday and today absolutely no noise. The only difference between the days it made noise and last couple of days is temperature. Last couple days night time lows in 60's and high around 70. On days when it made noise lows around 30 and highs around 50.

Any ideas? Thanks

 

[04_Sport_LS]

IF it's the alternator, then one of your accessories may be putting extra load on the alternator. Keep in mind that when in defrost mode the A/C compressor will run too.

Or possibly your blower motor is going bad.

There was one time I left the ignition on for about an hour.
When I started it up, I heard pretty much the same noise you describe.

After a couple days of driving back and forth to work... it went away.

Has the vehicles battery been checked. Sometimes batteries go bad without warning, which could cause the alternator to overcharge, which could cause the whining noise.

 

[JamesHinson]

IF it's the alternator, then one of your accessories may be putting extra load on the alternator. Keep in mind that when in defrost mode the A/C compressor will run too.

Or possibly your blower motor is going bad.

There was one time I left the ignition on for about an hour.
When I started it up, I heard pretty much the same noise you describe.

After a couple days of driving back and forth to work... it went away.

Has the vehicles battery been checked. Sometimes batteries go bad without warning, which could cause the alternator to overcharge, which could cause the whining noise.

Battery is 6 months old, no sign of weak battery. Everything seems to be working okay. Will see I guess.

 

[JamesHinson]

Turned out to be idler pulley. Still had originals, so was due.