I have a '98 Grand Cherokee 4.0L. When I start it in the morning (Mostly when its cold or damp) I get what sounds like a belt squeak. I've experienced this before on other vehicles, but the squeak keeps returning throughout my drive and the voltage meter goes down whenever it squeaks which I have never before experienced. Secondly, I have what sounds like a 'wind' sound (steady high pitched whine) whenever I am going above 20MPH it's driving me NUTS, but I cannot for the life of me locate it...Possibly a vaccume leak?

Any suggestions would be greatly appreciated;

Thank you,

Scott

Loosen the belts and spin the alternator by hand listen for squeaks. Also spin any idler bearings and listen.
From your description it sounds like your alternator has bad brushes rubbing against the commutator. If this is the case look for a re builder as they sell brushes.

Buy a new alternator instead if it is this,waste of time replacing the brushes and a better warranty.It could be a bad bearing in the alternator too and bearings are not available for most alternators.Tried it once even cross referancing the part number off the old bearing and parts store showed no bearing on the computer.When is the last time the belt has been changed?If not, look for cracks and sounds like the rubber is breaking down.Most new surpetine belts are a lot better now,made out of a much tough rubber these days

I'd start with checking the serpentine belt for wear. You said you lose voltage when you hear the squealing sound.

Do you have an electric or engine driven fan? If it's engine driven, it could be the source of the noise. Note what your engine rpms are when you hear the roaring sound, and have someone keep the engine at that speed while you listen under the hood - this should help you find the source of the noise more easily.

You should as pointed out check with belt loose alt and idler bearings. But a veh over 10years old I would buy new belt and replace. On your 98 4.0 you don't have a auto tensioner (came out in99)you need to adjust belt tension by moving power steering pump. The pump bolts need to be loosened then a long bolt at pump will allow you to swivel pump so as to tighten or loosen your belt. good luck