idling noise
 

my '98 5.7L transom is making a constant whining while in neutral. I first thought the bilige pump had turned on, but the sound was just a bit louder than the bilige. When I engage the motor the noise stops. It doesn't change when you turn the steering wheel. It doesn't change when you raise or lower the trim.
Any thoughts?

Re: idling noise
 

If you are saying that the whining noise disappears once the outdrive is placed into gear, my first thought would be simple gear whine due to gear lash. The lash is the space between the gears. Once the outdrive is placed into gear, load is placed on the gears and they'll stop whining. Boats that dont get used much tend to develop this noise over time.

Just as a precautionary measure, have you checked your outdrive reservoir fluid level? Also check your outdrive bellows closely for any splits that may have allowed water to get into the upper bearings/u-joints. Bearing noise though is usually constant. For my money, my bet is that this is gear whine and not bearing related.

Re: idling noise
 

my reservoir level has been good - I checked that a couple of times. I started to let out some of the oil in the outdrive and it came out with a much lighter color - after a couple tablespoons worth of "milky" looking oil it changed back to the 90 weight oil that I'm used to.

Re: idling noise
 

Bummer:(
You have water intrusion!
Bad seal in the lower unit at prop shaft is the usual culprit , but could be other places too.
Get it fixed ASAP , you'll destroy the drive if you don't.

Re: idling noise
 

Yep, as Larry says with the milky look of the outdrive oil you have some water intrusion that needs to be fixed asap before it causes some major damage. While you have the outdrive apart you might as well have a close look at your gimble bearing, u-joints etc to see if they've suffered any pitting from the water.